On a massive tour, it has to be 35mm Marathon plus for sure. Not sure I would want anything bigger, but lots do I guess! Better for carrying loads, better for being puncher-free. I don't want that hassle in the middle of France! At home unloaded using my Mercury for everything, I use the 32mm Specialized All Condition Armadillo Elite. Great puncher protection, plus they roll really well. I love the feeling of these tires. I have been using them for several years. I was shocked by just how nice they made any bike feel that they have been fitted. They are not cheap either, I have even used these on sportives in the past, I have had these up to 45k on a decent flat, 70km downhill. They roll just a little slower, but with 1 flat in 8 years, I keep using them. The Mercury runs amazing with these on.
Welcome to the club Marc!. I too spoiled myself for my 50th albeit with a non-Thorn Rohloff hubbed bike. I wasn't aware of Thorn as a brand, only discovering this forum later.
Contrary to popular folklore, wider tyres have been proven to have less rolling resistance than skinnier tyres. And not only that, they add some additional comfort in absorbing bumps and potholes much better, so that longer tours are more of a pleasure than a pain... literally!
I personally swear by Marathon Plus tyres. Most of my family bikes have them. Admittedly, slightly more sluggish than same size Big Apples, but hardly noticeable in the real world of being out and about on a bike. Any bike I ride tops 100kg+ with me on it. The heavier the load, the greater the risk of puncture. Lighter riders can probably afford to cut corners on how puncture resistant their tyres are. There is an optimum tyre for everyone, and this depends heavily on your riding habits, your total weight and the features that are important to you. As a heavier set cycle commuter hitting 160-200km a week and on a tight schedule, I don't want to be late to work because of scrimping on puncture resistance and getting a flat. Add in a few potholes and kerbs on my commuting route, wider tyres help absorb those unavoidable impacts. But that is me- and my circumstances.
There is an interesting comparison of Marathon "Green Guard"
https://www.bicyclerollingresistance.com/specials/schwalbe-marathon-32-37-40-47 of resistance across a range of widths of the same model. While it is not automatically comparable to other models in the range, it does show tendencies that go against the skinny is better mindset.
The website does allow you to compare other brands and models of tested tyres against each other to help you make your own decision finding the ultimate tyre for you. Rolling resistance is just one factor you need to consider. Comfort, reliability, handling... you need to juggle what is important for you before investing in a decent set of tyres which will hopefully give you many thousand kilometers of cycling pleasure.