Sounds like you have a good attitude towards the project, that helps, cause full Neander can be challenging on projects like this. Not sure your experience level, so maybe much of what I write, u already know, if so, hopefully it will help others. The other issue is, you can get both sides of the board FLAT, but not co-planar to each other. That makes for a funky glue up
Specially if you want cross boards on the bottom. There is all kinds of tricks to try to keep the board co planar which I am sure you aware of. Using a FLAT work surface is very critical here as well...
It truly is amazing how fast and accurate a powered jointer and thickness planer is. It is one of those tasks after I complete them for longer stock, I sometimes marvel how did ww do this 100+ years ago by hand! All day long, thats all they did. They prob had additional tricks n tips that got lost through time. I am Neander more on smaller projects, where I feel the tools fit the wood size better. A jointer hand plane works great on a 20" long board
Although maybe some time in the future, I will challenge myself to see if I can get a rough board your size, flat and co planar.
Yes, the wood will move... not sure how thick the boards are you are starting with...but as you know, as you remove thickness, it moves the most as the outermost areas are removed. So doing the project over many days, paying attention to the movement is advised.
As for movement over time, the cut of wood is critical here. Safest bet, use Rift Sawn for long boards, they are remarkably more stable than quartersawn and surely plain sawn. Of course, a finish that is equally applied to the entire surfaces also can greatly reduce movement. I have walked into a lot of antique stores, and no straight edge was required, excessive movements on some table tops. OTOH, I have seen some 400 yr old pieces done right, and movement was minimal. It really takes a LOT of planning at every step to achieve the end goal in ww. The larger the project, the more obvious movement becomes as errors double with doubling size.
Why doesn't everyone use Rift sawn? 1) not always easy to find, and when u do... 2) in many species, extremely expensive vs. plain sawn, sometimes 3-6x the price per BF, as the waste is so great and labor much higher!