I would argue some ways to design epic levels or godhood are simpler than others, more accessible than others, less book-keeping than others and easier to balance than others.
To some, that might constitute 'better'.
But the truism is you cannot please all the people all the time.
In your opinion what level would a quasi-deity be or be comparable to?
To me it’s about mortality. There are some ways to not die of old age (Monk, etc), and Chosen of Mystra also gave immunity to aging in 3e. But not dying of old age is different from immortality.
In my mind, it should only be possible for a god to kill another god. Or alternatively, in some settings/mythologies, if a god loses all of their followers and believers, then they might "die", or transition into some kind of dormant state of "forgotten god" in which they lose all powers. But regardless of the details, an epic level character, no matter how powerful, can at most be a thorn in the side of a god, thwarting their plans, etc, but never a mortal threat.
As for demi-gods, I guess that depends again on the mythology. Maybe only demi-gods and higher can kill them. Or maybe a mortal in very special circumstances has a shot at doing it too (maybe requiring indirect help from a god, e.g., given the information that only the heel wasn’t dipped in immortality oil). But in any case, it shouldn’t be a simple pedestrian matter of dealing enough damage.
Besides mortality (or lack thereof), deities should have gradually increasing ability to grant powers to their followers. As deities grow in divine rank, the scale of followers they can support should grow also. But as they grow in followers (both count and potency), there is a bit of plot force that pulls them away slightly from the realm of mortals. They should act directly only rarely, and instead effect the world through the actions of their followers. It is hard to model this via rules, and is more of a story-telling thing the DM needs to take care of. Again, this is setting-dependent, but in general, I think it makes sense.
No real reason for it though and if we were building him in 5e he'd be a level 20 Wizard most likely.
Well, that was his backstory, as far as I remember. Ed Greenwood wrote a novel on it I think. If I recall correctly, he started off as a thief (2e Rogue), and was a brigand or whatever, then became a fighter (or the other way around, not sure), then became a cleric of Mystra and only after all that did he finally become a mage, and not just any mage, the greatest mage the realms had ever seen.
With 5e rules, you could be rogue 1, fighter 1, cleric 1, wizard 17 and barely attain 9th level spells, so the canonical story is still workable, but he can’t spare a single level outside the wizard class otherwise he would be barred forever from learning 9th level spells and I think we all agree an Elminster build without 9th level spells would just be incredibly dumb.