This would be one solution but it wont work. No one is going to buy a tractor from some "open" manufacturer that they have never heard of or pay extra for the "open" tractor. There are a number of factors at work here. 1) Emissions ratings that the manufacturer must meet, 2) Vendor lock-in (and therefore fleet lock in), and 3) Pride. You see out here in the midwest folks are mighty proud of the color of their tractor. It can start an argument faster than vi vs emacs. To come into a market dominated by big players will be a challenge. Especially when there is both manufacturing AND mindshare hurdles to clear.
The easier solution was already mentioned in the summary: support the Right to Repair bill, in any state, if your state has made one. This doesn't overnight solve the problems and John Deere would probably just pull a TiVo and lock the software and it is business as usual. And expect pushback from the vendors when it comes to these new bills but it is the first step to taking back ownership rather than "licensing" your use of a tractor.