even for the 3090, I am assuming you are trying to run max settings at 4K then? not gonna happen at 120fps.
4K is still super hard to run at 100fps or more on extremely graphically demanding games like RDR2 at max settings.
you don't need to go medium.
You have a couple options:
Lower the preset from max/ultra to the next level down for graphics.
Fine tune the graphics settings to lower a couple of options down to next lower level.
If you are running Ray Tracing don't. this is why you get less FPS than you think you should.
If you are running ray tracing, use DLSS if the game has the option for it.
If you are running ray tracing, lower the ray tracing quality.
88fps is nothing to laugh at. it's more than smooth enough for a 3rd person open world game.
It's not like running less than 120fps is going to make it a bad experience.
Suffice it to say, if you are trying to run max settings at 4K and most likely with ray tracing, you need to either lower the ray tracing quality, or lower a few graphic settings from max to the next option below max.
Specifically options that have the biggest impact to frame rate such as lighting and shadows which is why I mention lowering the ray tracing quality if it's available in that game (I wouldn't know as I bought that game for PS4 Pro when it released rather than waiting for the PC version)