i just want to be able to get 240fps 1080p on most games
In that case you want to invest heavily on the CPU side of things - very high frame rates require high IPC + high clock speed CPU.
A faster graphics card will allow you to hit high frame rates at higher detail settings, but the CPU is arguably more important. Also keep in mind, there are many games that simply cannot hit these kinds of frame rates as the game itself isn't optimised to do so (for example it's literally impossible to get over ~ 100 fps in the new Microsoft Flight Simulator, no matter what hardware you have). The types of games which are optimised for very high fps (fortnight, CS:GO, Dota etc) are usually pretty easy to run graphics wise so tend to favour high performance cpu over the gpu.
Currently the best CPU choice for 240 fps games is an intel 10th Gen cpu (Core i5 10600K or better - needs to be a 'K' series as you will want to overclock the CPU to over 5ghz so also needs a pricey Z series mobo and good cooling).
The AMD Ryzen 3000 series CPU's can hit these kinds of frame rates in some titles, but these cpu's can't reach the high clock speeds Intel can (they top our ~ 4.3 ghz all core, although they do have other advantages over 10th gen like larger Cache and higher IPC in some cases so it depends on the game somewhat). These would offer an easy upgrade for you as you can keep your existing motherboard. AMD are due to release the next gen Ryzen 4000 series desktop cpu's which are supposed to be more competitive in games, and B450 motherboards will be getting an upgrade option for these cpu's although you may have to wait a while based on AMD's announcements about it.