I’m currently thinking about upgrading my 3700x to a 5900x. I have a 6900xt currently which is used for maxed out 1440p gaming and perhaps 4k in the near future. Will I see a massive fps increase if I change cpu? what sort of performance boost/advantages will I see?
Longer response, normally I'd say it's not worth upgrading a 3700X because it's still a very good CPU and more than enough for most peoples gaming needs. However you have the top end premium GPU, if you want the full premium experience it makes sense to pair it with the premium CPU.
In terms of the benefits of the upgrade. If your playing at 1440p at high refresh rates, assuming at least 144hz, then I think you would notice a difference on games like Battlefield. Your frame rates will be higher, it will hold them more consistently and will feel smoother. I would say there is a perceivable difference in Cyberpunk 2077 beyond 8 cores as well.
It won't be like going from a 4 core to a 6 core but there are situations when a 3700X will hold back a 6900XT a little. The 3700X is fine, but if fine was good enough you probably wouldn't have spent so much on a 6900XT