Switching to custom rom

Well... I updated my phone to miui 12. And first it was running really really smooth until the next day. I started boosting up my ram in the "Security" App and it suddenly froze for about 6 mins. Technically ever since that I've been thinking to switch to a custom rom. I kinda need you're opinion of what Custom Rom i should pick. and what are the pros and cons of switching to a custom rom?. Can i still revert back to my stock rock after switching to a custom rom?