How to get started

I don’t know about the canbus… The aio 4.1 has one canbus chip. There’s a few tractors that require communication on two separate busses, on those a different canbus board is required. Might find some help here: CanBus for Beginners - Super Simple - #1218 by hnvcad

Trimble can receive (or some can) RTK from your base station. Like was said, you need message 1008, included by default if you go with Stefal’s RTKBase.

I don’t think so. The enemy of open source side projects, time, seems to be the issue. Brian and co. have been busy getting v6 out before spring work starts.
This is getting out of my territory, but I’ll say what I know. If you have some spot to add in a relay to trigger a section on/off section control will work. And depends what you mean by “planter control”, there is a rate control program, and a planter blockage monitor in the very beginning stages of development.