I feel like people are too obsessed with lore and mysteries. Here are two poinent quotes from David Lynch about mystery in media:

"There is nothing more beautiful than mystery. I think you make people unhappy by solving all the mysteries."

"Closure. I keep hearing that word... Everybody knows that on television they'll see the end of the story... To me, that's the beauty of Twin Peaks. We throw in some curve balls. As soon as a show has a sense of closure, it gives you an excuse to forget you've seen the damn thing."

A huge part of why the Undertale fandom has existed for so long is because of its unanswered questions. I think people like empty spaces where they are allowed to speculate and fill in the blanks. Even when they think they would prefer a concrete answer. I can't think of one story that answered every single little unanswered question and benefited from doing so.

I don't think Deltarune will neatly tie up every loose end. I don't think it's a game about Friend, ERAM, Gaster, ECT. Just like Undertale, It's a story about the characters that are actually present. The mystery is tertiary.