Typically what you'll find on the level higher than greater gods are overdeities, which is a hilarious power difference considering the latter have no stats and are >>>>> all other deities, having control over the entire setting and putting them decently high into multiversal. Shit like the Lady of Pain, where the only rule when fighting them is 'you lose, good day sir.'
Elminster definitely should be on the level of at least lesser gods but based off his "I am better than you" status with Forgotten Realms I wouldn't be surprised if he was greater god level honestly
Levels are a bit different with each edition (mainly from 3e onward) as only 3e and 4e actually allow characters to reach epic levels. 3e technically had no level cap and 4e capped out at 30 but had the occasional CR 35 enemy intended to be thrown at a max level party. 5e did away with epic levels entirely but also introduced epic boons which are intended to be given to level 20 characters so the intent is still there. But usually a five level difference is enough to go from a 'modicum of challenge' to 'stomp fight', I just wanted to explain what the hell is going on with levels (and why I haven't brought up Eberron)
Since the lore is otherwise the same the best way for me to go about the changes is to just adapt the scaling for each edition based on any major figures, but apart from 5e usually being a bit fucked it seems to be alright.