> Announce the Prefix of the currently active connection, invalidate the other one

And this doesn't actually work. Prefix deprecation is a best-effort feature that is not implemented correctly in tons of devices, including such rarely used niche operating systems as macOS. It even technically violates RFC4862 (section 5.5.3).

As usual, IETF only recently woke up to that reality: https://datatracker.ietf.org/doc/html/rfc8978

I highly recommend actually trying what I proposed. Not in a theoretical hand-wavy way, but actually setting it all up and verifying that it works. I did not pose this challenge in a "gotcha" way. I really was not able to make it work cleanly with either Mikrotik or OpenWRT routers.