The 90D is a crop sensor camera. That means the sensor is smaller than a full frame sensor (like the RP).
Look here:
The 90D has more pixels but on a smaller sensor. The RP's full frame sensor has fewer pixels on a larger sensor, so the pixels are bigger and therefore better able to sense lower light levels.
Hence why full frame cameras are (generally) better in low light.
If you are going to go full frame and change mount for the RF mount, I'm not sure I would bother keeping any EF-S lenses.
If you put the RP into crop mode with an EF-S lens you'll only be getting around 15mp, because the EF-S lens can only put a small image circle onto the sensor and the camera just ignores the pixels around the edge.
Besides, what EF-S lenses do you have that are going to be worth keeping?
I did the upgrade from crop sensor 60D to full frame 6D. I sold my EF-S lenses to pay for the upgrade and kept my EF full frame lenses. The EF-S lenses wouldn't work on the 6D and even if they did, the full frame camera would show up their deficiencies.
I ended up just keeping a 50mm, 24-105L and a 90mm macro. Since then I've built up again.
I am currently thinking of upgrading my 6D to the RP, but I'll be looking to do the same again. But likely sell all my EF lenses and buy the RF lenses.
I tested all my current lenses with a hired RP and mount adaptor and although they all worked, the adaptor makes life faffy and adds to the bulk of the camera/lens combo, which is something I want to avoid.