You can choose any realm but Firemaw. Classic Era and Classic Season of Mastery are becoming more and more empty so play on TBC.
Yes, you will miss the Classic Era raids but you probably wouldn’t be able to do them anyway as they are only guild runs or runs you pay for now on the vast majority of realms.
Realm choice is important but bear in mind that Blizzard are closing down a lot of almost empty realms in August.