Sorry in advance for being a downer here.
I'd encourage you to look at how we might move the existing backend forward rather than attempt a rewrite. The reality is that no existing extension will be compatible with your backend. This makes it a non-starter for 99% of stores. Fragmentation of the community is one of the issues we sought to solve with Solidus. By keeping the community we all benefit from the improvements that get made to the platform.
You've got good ideas in there, some even that we've started to explore, like modernizing the JavaScript technologies in backend and bringing them in line with the latest Rails defaults. You're welcome to go your own way on this, but I'm sure the community would be more appreciative of effort put into improving what we have already than something that would require every store and extension to rework their backend customizations.