I have a 3080 Ti with three monitors. The main one is 4K and the two side ones are only 1080p. The card handles it well and I seldom see it use all 12 Gigs of ram. At about 11 Gig it starts using system RAM, which is not a problem, other that it being slower, since I have 128 GB.
It will only fill up In big photogrammetry cities when running Ultras setting and rendering at 4K. If you use or render at 1440p (2K) or 85% of 4K ( which is 3K) or run on High-end setting, you shouldn’t run out of VRAM and using RAM.
I’m very happy with my 3080 Ti. But if the 3090 isn’t priced much higher, go for the 3090. Besides the extra VRAM, it does perform just a little bit better.