"But just by waiting 6 months, I managed to obtain the EOS R at its lowest price ever."
You must put some value on using the R or you would not buy one. The 6 months you lost waiting could be greater value than the money you saved.
"Upgrade lenses first, then by the time you get to upgrade the body, you will get it at a reduced price, win-win."
No this is not right if he wants to get RF and not EF lenses. Why would someone want to buy new EF lenses than the R? only to replace every EF lens with RF at some point? The focus alone is enough of a reason to upgrade not to mention the sensor and I am finding the FV mode very compelling. NO EF lenses with an adapter are NOT the same as RF lenses.
"The only real reason to use crop lenses on these is to use the cropped 4k video modes with a wider angle of view."
Or because you already have a set of crop lenses.
I think you are barking up the wrong tree.
Perhaps re-read the thread from the beginning. The OP made it very clear that the intent was to use EF-S lenses at this stage, with an aspiration to get good EF (or perhaps RF) lenses later on - at least I thought so.
Hence my comment that it made little sense to downgrade from a 18Mp APS-C body to a 10Mp APS-C body. With EF-S lenses the RP is NOT a FF body - period. And I believe (and I am happy to be corrected) that the OP wanted a FF body to use with the EF-S lenses.
I did not say that it was not possible to get a decent photo with a 10Mp camera - of course it is - I owned them for several years.
But if the OP thinks that buying a still expensive 26Mp FF is going to produce 26Mp FF images with EF-S lenses, then that is an incorrect assumption and somewhat of a waste of money unless the EF-S lenses are going to replaced quite quickly with EF or RF lenses.
Colin
Using EF-S glass is a viable solution if you dont make huge prints or crop heavily.
1- You get a FF camera simply to have the OPTION of going FF when you need/want to, as in, shooting with a 10-18mm like I used to do on my 12mp 1100D, you get the same experience but with a bit better IQ & a much better EVF, much better noise performance, and you have a FF sensor whenever you want to put on that 50mm to get FF shallow DOF.
2- If you do video. 4K video on the EOS R & RP is an APS-C crop, and using FF glass is wasting all these large optics. The 10-18mm is the only solution for UWA 4K video on the EOS R, giving a 16-35mm IS STM lens.
3- If you own third party EF-S glass that covers FF. You'd be amazed on how many crop lenses work wonderfully on FF chips, giving the full 26/30mp resolution or at least 24-25mp after cropping the very edges. Examples:
-Tokina 11-16mm f/2.8 is a 16mm f/2.8 Prime on FF
-The Sigma 30mm f/1.4 covers FF wonderfully
-I just made a thread on the Nikon 35mm f/1.8 working brilliantly on a FF 24mp D610
-Sigma 18-35mm f1.8 is a 24-35mm f/1.8 FF lens
-Canon's 24mm f/2.8 nearly covers 90% of FF
So it's about having the FF sensor as an option when you need it. And you also get things unavailable on any APS-C camera, like 4K recording, clean 10bit output, C-Log internal, the large 3.9m dot EVF, large battery & grip, briiliant Wifi implementation (it's the one feature I like most about the R, I press the shutter and go share on the phone, it's too slow on the Crop Canons), it's weathersealed, has a top Oled screen, has three dials, has Eye AF in and works in Servo & video,
You're not just paying for the larger sensor nowadays, these are fairly cheap to produce in 2019.