I'm still relatively new here & I have absolutely no idea how all this server stuff actually works, but if I may...
Rather than just deleting the "oldest" accounts, why not delete all the "inactive" accounts?
Say, like, any accounts that have remained inactive for 1 or 2 years? Are y'all able to look into that?
I imagine there are some people, other than the original moderators, who have been here since day one & would be pretty upset if they were to try to log on, after y'all delete a bunch of older accounts, & find that their account has been deleted just because it's old. Whereas, someone who hasn't been here in quite some time, & tries to log in with their old account, wouldn't be quite as upset to find that their account was deleted due to being inactive for so long. They could then just easily make a new account, if they decide to become active again.
I would think 1 or 2 years of inactivity would be a reasonable amount of time to let an account remain inactive, before it gets deleted by default. ^^ Why not start with those & see how much it helps? This might be something to make a regular habit of doing, depending on how many dead accounts there are. The more dead accounts, the more deleted accounts - The more space for the active accounts & more space for the server to run more smoothly, right?
I hope this suggestion helps. ^^ ~Eimi