Since you get the same performance at 1080P and 1440 it suggests to me that you do have a cpu limitation.
As another test, in windows power management, set the max cpu down from 100% to 80%. If you are not cpu limited, you should see no change in performance.
Assuming you want a cpu upgrade, what are the options.
1 ) A I7-9700K would gain you two threads and perhaps 20% better single thread performance. Possibly more with overclocking.
That will cost about $220 used on ebay.
2) Buy a current gen amd or intel processor.
The $200 range will give you a big boost.
The thread count will double, and the single thread performance will go up by 50%. In general, amd gives you more threads, but at a comparable price, single thread performance is a bit less.
A I5-5600x is relatively cheap at about $168 and is exactly comparable in performance with a I5-12400 at $179.
Less expensive 13th gen processors are due to be released soon and will offer a 15% boost in price/performance.
If you can wait to January, that would be a good deal.
In either case, you will need to buy a new motherboard.
B660 may cost $130 . I am not expert on amd chipsets but would expect them to cost the same.
With Intel, you can reuse your DDR4 ram. Intel does not depend much on fast ram. Reusing is uncertain with ryzen. Not only is ryzen dependent on fast ram for performance, but it is quite picky about compatibility.
A cpu upgrade is a start, buy it may just swap your limiting factor from cpu to gpu.
You may want to check the warzone2 forums to see what is required to get to 100fps.