That's the biggest issue imo about old Trek, and shows in general from that era. We are so used to watch on demand now, that we do not miss episodes. But back then it was mostly written that if you missed a few weeks, you could still jump straight back in. So it was mostly written with the idea that you did miss a few episode, and either making no references at all, or rehashing old stuff as if you as a viewer already forgot about what happened.

So in this case, if they had ee-Char show up every so often, they would need to explain this to the audience every single time, just like why he changed so much.

I definitely missed this with Picard as well when he became Locutus. An absolutely tragic and devastating thing, he would normally be messed up for a long time, you would constantly see him struggle etc. In a modern show they would probably make that last like half a season, have Riker and Troi support him constantly to both keep going, and not look weak among the crew. But instead they gave him 1 episode to sort it, and after that some references here and there.

DS9 was already quite rare because unlike TOS, TNG and VOY, DS9 had constant storylines going on, on the station with many (recurring) characters, you had all these races like Bajorans, Cardassians, Klingons, the Dominon races, Ferengi etc etc. And sure, TNG had some of that, but it was very minor, it was still very much focus on the main cast and the guest stars that only showed up for that episode, usually never seen again.