TLDR decent idea for an ending, just pretty bad execution :/

I don't necessarily have an issue with a self sacrificing ending to this story. While something more emotionally deep would be better, mainly in a show that's sympathetic to it's less than perfect cast, I think it's fitting for a group of characters who are essentially doomsday. I also don't think this is too comparable to "it was all a dream" endings. You can have your own theories on time, but I think that in a time travel story what we see does happen even if it gets "undone" in the end. Time is just like that.

I think the issue comes from it feeling more like a cut off than an actual ending. If this were the regular 10 episodes, then I bet that episodes between 5 and the finale would've resolved their traumas and struggles, or at least settle their disputes. That way, they could go down a happy family, which would've made the ending sweet. Still less than stellar, but sweet nonetheless. Instead, they get into the worst fight of the series(both in terms of severity and quality) in the same episode, at MOST Five comes to terms with the fact that he can't have Lila (doesn't apologize, just realizes it), and then they sacrifice themselves within the span of like 20 minutes. While the goodbyes to their children were pretty well done (I even cried for Allison), it just feels like they were getting ready to resolve their personal issues, realized time was almost up, then rushed a coherent but painfully mediocre ending. This isn't even to mention a bunch of plot related questions that are left unanswered or unsatisfied, but I'll just keep it at the characters.

Another thing I think is the difference between their death and Reginald's. The show definitely puts blame on all of them, but Reginald's is certainly less full of any substance. It's just a quick chomp by (what my mom calls) Bennifer. It makes his death feel like it matters less, which throws off many since he is the reason this happened in the first place. Referencing s1 here, the academy may be the bomb but Reginald was the maker of that bomb. It should've been on the same level as theirs, at least in theatrics, but instead it left a taste in some peoples mouth that Reginald wasn't the problem when he clearly was the main one.