Solo. Not that shocking.
Bike touring does require a lot of skills, not sure how much background you have with camping, campsite cooking, bike repairs, etc. These activities take some skill that are best learned before your big adventure bike trip. I have heard of people that almost never camped, never worked on their bike, and decided to do a bike tour. Sometimes that did not go well. I did a lot of backpacking, canoe camping, kayak camping, etc., before my first bike tour. I worked in a bike shop before I went to college and got an engineering degree. I built up my touring bikes from parts, laced up my own wheels etc. I am not saying that having these skills is a requirement, but they made my start in bike touring a lot easier. That said, I found it easier to do my early tours with a friend so we could talk to each other about whatever came to mind. After several short tours with others, solo touring was pretty easy.
Traveling solo, if you are flexible for schedule, that is great because you do not have to have a meeting with proposed changes. If it is cold and rainy and you want to check into a nearby hostel instead of camping, no discussion needed, you do it. If you complain about the food, that is your fault. You are in charge and if you do not like what the boss did, too bad.
Traveling with a friend, works best if you both are about equal in speed and desires for distance per day, both have similar goals for how much camping vs indoor sleeping. Advantage is you can share camp chores and other tasks. The friend that I have done several bike tours with does not like to think about the navigation issues, etc. This is a bit inconvenient for me, as we travel together because he lacks a GPS. Thus, he has no clue where we are going. I do not mind the trip planning and navigation, but it would be nice at times if we both could ride separate. The group tours I have done with others, it is very common to find most people are riding alone even when they all are in the same group that night in the campsite.
Flying is no fun solo, if you are at an airport and have to go to the loo you are supposed to keep your luggage under control, etc. But with a friend you do not even think about how many times one is watching everything while you are off doing your thing. It is very easy for stupid arguments to start up because there will be disagreements. The worst argument is how much work the other does and did they do their share. If you have a goal of doing 20 percent more of the campsite chores and other tasks, then if your travel partner only does 10 percent less than you, you still did less work than you anticipated and have nothing to argue about.
I am single, no spouse, live alone. So I am very used to doing my planning for everything solo, so a bike tour to me that is solo just makes sense. If you live with someone else, when you are off on your own, that will be a culture shock when you are suddenly making all the decisions based solely on what you want or don't want.
Depending on where you are going, security of your gear when you are solo can be much more of a concern than when you have two sets of eyes on everything. My solo trips were to places where theft is not much of a concern, but I have read of and talked to other bike tourists that have had theft issues cause problems with their trip. One woman I talked to was on an around the world tour, started in the UK and went east in Europe. Only a few thousand km after her start, her bike and everything on it was stolen at gunpoint in Eastern Europe. So, she had to start from scratch, her high end touring bike with the good stuff was gone, so she was riding the rest of her around the world tour on a low budget mountain bike with solid fork. I met her in my community, I was out on an exercise ride and saw someone riding with a brand of panniers not sold in USA, so I started up a conversation with her. By now she had made it through asia, flown across the Pacific and was half way across USA.
If you live with a spouse or partner, I suggest you first do a one or two week solo tour near home first. That way you will understand how to do a solo bike tour and make your decisions solo, something that you are not used to doing.
List of my tours below. I wrote this for the benefit of the OP. I expect others will find this boring, sorry.I did my first tour about two years before I retired. Did that with a friend of mine, he sat in the cube next to mine at work, he had done some credit card tours with his brother before. I wanted to tour with camping, not sleeping indoors each night. We both worked at the same place, did similar jobs, had a limited number of days of vacation available. So we did a four day tour, add two days for driving to and from. Had a great trip, was a new type of experience.
Next tour, we had both retired by then. I think that was a six day tour, plus two days driving.
Did two guided and fully supported trips in Europe, they provided the bikes, most food, lodging, etc. Week long. I do not call this touring, but some people do. I went solo, some in the group of about 8 to 10 went solo and some with a spouse or partner.
Third tour (or fifth trip if you count the above), I did with Adventure Cycling Assoc. I am in USA they run a lot of tours. About a dozen people. Had a great time, but the food was not very healthy for me being a diabetic. On this trip, the friend I had toured with before did not attend, so I traveled to and from the start solo, but was with the group for the week long trip. I hate to go a long way and then rush home right away, so I went there three days in advance, camped in a campground and did sightseeing.
Fourth tour, this was actually the locale that we had talked about first and had started preliminary plans years earlier, but it was far enough away that it was a train ride to and from. We let a third guy join us. He was retired, had been a supervisor where we used to work but neither of us knew him from work, but he had crossed the country on his bike with a group and wanted to join us so we let him. He immediately tried to start changing the plans that we had made a few years earlier. And he had that supervisor mentality where we were supposed to follow his orders. Bottom line, none of us enjoyed the trip that much.
Fifth tour, ten of us from my community, most of us knew most of the others in the group hired an outfitting company to haul our gear, food, and water. And fix our food. We planned the trip, got the permits, etc. White Rim Trail in Canyonlands National Park. The outfitter was a tour group company but since we did the planning, permitting, and had a guaranteed group size, they charged us about half of the typical cost.
Sixth tour. With my friend and former co-worker. Pacific Coast from Astoria Oregon to San Francisco. Total with travel to and from, and sightseeing time, about five and a half weeks. Most of it camping in State Parks. This was our first multi-week tour, but we were both retired do multi-week trips.
Another trip, I won't call it a tour, the former co-worker and I did four nights and five days of mountain biking in a national park, camped at campgrounds that we drove to, so the riding was only carrying our food and water for the day. Initially we planned to ride the trail with our camping gear but we were there about two hours and the trail was tough, we changed our minds fast to doing car camping instead. More here:
http://thorncyclesforum.co.uk/index.php?topic=11321.0Seventh tour. Solo. First international tour (not counting the tour group company tours I did in Europe listed above). First tour I flew to. Iceland for a month. Had a great time. More here:
http://thorncyclesforum.co.uk/index.php?topic=11917.0Eighth tour, two weeks of biking and some time to get there and home again. With former co-worker. Florida in USA, Everglades National Park, Florida Keys.
Ninth tour. Five days, four nights rode from my home and back with a friend. I had built up a new touring bike (Lynskey), and he was testing his upgraded bike and new tent before a cross country tour, he had toured before.
Trip with ACA in West Texas for a week.
Tenth tour, solo, international, a bit over five weeks on the bike, add a lot more sightseeing time. More at these links, the second and third links are not trip specific but I wrote them up after this trip:
https://www.bikeforums.net/21306368-post40.htmlhttp://thorncyclesforum.co.uk/index.php?topic=13407.0http://thorncyclesforum.co.uk/index.php?topic=13696.0Then covid happened, have not been on a tour since, but am planning two. I am now 68 years young.