Try
Settings > Device care > Memory
And where it says RAM, press it and then select the highest amount of RAM possible.
If still doesn't work, check if you need to do any software updates.
If still not fixed, I'd suggest a factory reset.
But tbh if I was in your position, I'd just get a new phone now. I had the S20 FE myself until 2 weeks ago when I got the S24. You should be able to trade in your S20 FE to get around £190 OFF an S24 as well (from samsung store)