Not being interested in linux, Haiku may be the killer app behind bringing RISC-V to the Reform. Its devs have coalesced round the VisionFive mk2 which happily uses this chip (as does the Pine Star64). Haiku never really got going on ARM architecture for various reasons.
So perhaps Milk-V Mars is a Haiku compatible option that can be done (almost) straight away for development purposes?
As for a “standard fitment” RISC-V solution, my only concern is that RISC is still quite fluid and the niche OS like 9Front or Genode may be in a position where they can only support a handful of SOCs well. And they work happily on the existing i.MX8M. So we would want to be confident that the RISC-V SOC we adopt is likely to have a long service life with headroom to grow, and enough of a performance uplift that these OS benefit from migrating over. Might it be worth seeing if StarFive bring the new generation of HiFive cores namely the 600 (BIG) and 400 (little) series onto a SOC in the near future?