I seriously regret selling my excellent copy of the 100-400mm II so that I could help fund buying Rf glass. Seriously.You're right about that! I was barely able to sell my EF 16-35 F4 for a decent price...BTW: that's why I am eagerly waiting on the RF 14-35mm F4 or anything similar.
At the moment, I only use the EF 100-400mm with an adapter, so I never take it of and it doesn't bother me.
Jumping of board the EF glass and getting RF glass needs to happen in near future in order to get value for my equipment. But my question is this:
I bought the EF 100-400mm in 2019 for 1.520 € brand-new, out of the box. Getting the RF 100-500mm would mean an additional cost of 1.500 € at the moment (plus the money I lose on my sale...). 1.500 € plus X for just 100mm extra range? Yikes...
I'd say that unless using it with an adapter just destroys the ergonomics for you, stick with your 100-400mm. Don't forget, the 100-500mm will need a new tele-extender too (if you use those), and that extender can't even be used for the full zoom range. Ugh.
While you do get an extra 100mm with the 100-500mm, consider that the R, R5, and R6 have crop mode built in, which would give a similar "extra reach" as using an aps-c sensor...I'm not sure that would be wonderful on the 20MP R6, but I think it would be very nice on the 45MP R5. (I haven't tried it on the R yet.) So you might save the money, try to get closer to your subjects, and just use that 100-400 until it don't work no more! OH HOW I WISH I HADN'T SOLD MINE!