Hi Derek
Crazy Guy on a Bike (
https://www.crazyguyonabike.com/) is the go-to source of information for touring in SE Asia (and the rest of the world too in my opinion). Restrict your searches to Journals and to the areas you expect to visit and you will pick up a ton of useful information.
We are currently in Vietnam having cycled up through Malaysia, Thailand and Cambodia from Singapore so we didn't cross into Laos this year. Last year we crossed into Laos twice from Thailand and entered Thailand three times (at the airport and from Laos and Cambodia). However, conditions do change and it might best to arrange a multiple entry Thai visa before arriving. This year we arranged Thai visas in Kuala Lumpur and Vietnamese visas in Sihanoukville but last year we just just arrived at the border posts and sorted them out there.
North East Thailand is mostly extremely flat close to the Mekhong but further away from the river, some of the National Parks in particular, have some good hills. I suggest having only a broad itinerary. Thailand is a very easy country in which to cycle tour and not much planing is needed.
We have spent four months out of the last sixteen cycling in Thailand and I am happy to field any questions you might have.
Regards
Jean-Mac