Roblox is a CPU intensive game and is heavily reliant on single-core speed (it doesn't use more than 2 threads). Even if you get a new GPU, you'll still encounter a lot of games that will cause FPS drops due to CPU bottleneck, an example of this would be Detroit RP (https://www.roblox.com/games/3119253190/BUS-TERMINAL-Detroit-RP).
So what's the solution? Get a new PC. I recommend the following specs because they're cheap:
GPU: GTX 1650