The 7D mk II and the EOS 5D Mk IV are both exceptional cameras of the last decade but I like the EOS R for most things much more with WYSIWIG EVF and an consent accurate focus within it's lower speed domain. I trust it for macros and I never again going to consider calibrating the range of of my DSLR focus for a lens with micro adjustments in the camera. It is not required with the EOS R. No loss of keepers from frontfocus with f1 or f1.2 lenses. It is a modern miracle and just works and the exposures come out consistently the way they look in the EVF before you shoot and and adjust the exposure a hair or two as shown.
Oh, that's another thing I didn't even mention, nor have most other comments, but I do once in a while enjoy try to get some close up shots with the lenses I have, and hope to get a 100mm f/2.8L to do some macro work with. The whole EVF showing you what the image will look like after taking it, compared to my 6D now, where you're guessing based on the settings and exposure, will be a welcomed update. As Jared Polin said in one of his most recent video's, the technology is here, use it!
I made the switch from 6D to EOS R two years ago. A few points not already addressed by others:
1) The M-Fn bar may or may not suit you, but even if it doesn't, no worries: EOS R has the same number of dials and buttons as your 6D, and the M-Fn bar and the touch panel are merely in addition to those. You do have to get used to having the dials in different places.
2) For moving subjects, any problem with EVF lag will be more than offset by far better AF tracking. Your 100-400L2 will be a lot more fun to shoot with the R than with the 6D, with or without the teleconverter.
3) The 6D's low-light focus capability was excellent for its day, but the R is on yet another level. Prepare to be surprised.
4) One bad thing about the R: when you shoot video or live view, you will no longer have time to go grab coffee while waiting for the camera to acquire focus.
Very nice, I'm glad to see I'm not the only one making the jump from 6D to R! Yeah, the M-Fn bar was very highly contested upon the R's release, and I'll play with it, if it is more trouble than it's worth, I'll just disable it. I'm just glad we have the option to do so. That's great to hear about the lag, not really being that much of an issue, especially when compared to what I'm coming from (6D). Yeah, I don't even use the TC because of the lack of AF on the 6D with it, due to no autofocus points. I actually have a lot of issues with grain in my shots at higher ISO on my 6D, but I feel like the issue is more of missed shots due to bad autofocus than it is digital noise, so I'm excited for the R. Hahah, I guess not having time to get and sip your coffee could be a downfall. Luckily, I don't shoot video ever, so I have no idea how slow the 6D is for that!
You will absolutely love the EOS R. I recently upgraded from a 6D to the R6, but also considered going for the R a long, long time. Whilst the R6 has more features to make your live as a photographer easier (that AF is sweet AF...) you will get 50% more pixels (which is an advantage for cropping in). Good luck in finding a good used R at a reasonable price.
Yeah, I'm excited about the ability to crop more than I can now, especially since I shoot mostly prime lenses, so sometimes I take a shot on something wider than I'd like too, just because I don't have the time to swap lenses before snapping the photo. So cropping will come in handy in those street shot situations when I've got the 24 on, but wish I had the 50!
I wouldn't change to an RP - I've looked at many 6D and RP photos closely on Flickr, and I think the RP's image quality is a downgrade from the 6D.
Changing to an R but keeping the same lenses won't change weight or size significantly.
If you change to an R the biggest difference I think you will see is focus accuracy for shallow dof shots, particularly portraits. Also some extra cropping ability.
So it's up to you. The R is not a big enough jump for me from the 6D, particularly not having ibis (that's why I waited for the R6, but it's too expensive), but if you took lots of portraits or wanted better video for hybrid shooting, then it certainly could be worthwhile.
I wouldnt get the RP as a replacement for the 6D, but it would be a fun camera to pair with a light, small lens and take out sometimes for occasions that suited it. It might be good camera for your girlfriend to get before the trip - lighter and better IQ than the 7D II, and then you'd be able to borrow it too for times where mirrorless could come in handy. Also, for situations where you both wanted to go out with as little weight as possible, going with only one RP and a lens or two, to share between you, could be very light and liberating.
I see others mentioned that the RP isn't a downgrade from the 6D, which I wouldn't imagine it being a downgrade, if anything parallel or better (just due to technology differences). But either way, I'm pretty much set on going R at this point, between the features, the MP bump and the cost (not R6 territory) it seems to fit the bill. I don't think the IBIS will really matter much to me, as I don't shoot video (as in, I've NEVER turned my 6D to video mode since I've owned it) and I don't do that many portraits to justify waiting for an R6. She loves her 7D II and even if I offered her an RP or R, I don't think she'd take it right now, lol. So, I'm thinking maybe late 2021 or 2022, when there's more R options, maybe a crop-sensor, as she likes having that extra reach, we'll look into mirrorless for her. Plus by then, I should have an RF lens or two.
Go for it.
I bought an eos R in April 2019 and I loved it but decided to stay with sony because of the better af (before eos r updates) and ibis.
I still missed Canon colors so I bought a cheap 6D (like 320 usd), but I only had it for a couple I'd weeks and I sold it again. It was like going back in time. Horrible af and metering, too bad dynamic range, etc. But gorgeous colors.
Recently I bought again the R and it's a great camera. Still missing ibis, but I won't pay the current R5/R6 price for ibis alone. Maybe in 1-2 years.
I get that, if you're into video or need IBIS for what you shoot, that makes sense - also, I'm a big IT nerd, so I'm always making sure my firmware's are up to date, getting the latest fixes and features

. I do love the colors of my Canon, but I understand why you would hate a 6D after using newer camera's. They're cheap - but for a reason, lol. Glad to see that you are enjoying your R, thank you!