As an enthusiast that takes portraits, travel, some macro, some wildlife, and a desire to do more sports and birding; I’ve always wanted to upgrade my SL1 to a full frame sensor/mirrorless.
Price is a big deal for me, and I want to buy once cry once–i want a camera that will last me the next 8+ years. Reviews on the RP look great, and I don’t pretend to be an expert–im confused though, because while the R6 mkii has a few nice features I can’t see why it costs twice as much. Is the R6 mkii worth the upgrade, and/or will it be more future proof?
M system is pretty much EOL’d, so it may not be the best choice for future proofness as the OP wants.
It’s a good point about lens availability, though. The best selection in the mirrorless world will be Sony, due to the system being more mature and Sony not being hostile to third party makers like Canon and Nikon are.