Libre Le Potato is also a really good alternative to the rpi3+/4. It has emmc support and is quite fast for $35 (without emmc). It's supposed to be on par with the rpi3b+ in terms of processing power but it's the emmc that is a huge boost. The GPIO pins are compatible with the rpi so you can definitely connect via uart.
I am using 400mm arms, so a 310mm diameter bed isn’t much of an issue. Biggest annoyance with a bed that large, I’d say, is trying to keep everything level
Haha you’ll figure it out. Start with tightening extrusions and belts and making sure everything is square (or triangle?). Also make sure that your z-offset is dialed in
