Looking for advice about a novel that's grown too long

I’ve been working for several years on a novel, and in a lot of ways it’s been fun. I have some very interconnected themes, some plot twists that tread the line between being surprising, and meaningful, and a fair few characters that develop through a lengthy confrontation.

I’ve started to consult an editor about tuning it into something publishable. Due to the way I was writing it, I only recently got the tools to calculate a total word count, and we realized that in the end, it’s far longer than I wanted it to be; on the order of 370,000 words.

Apparently people like George R. R. Martin can sometimes get away with this length, but I understand this is way out of line for a first time author. I’ve been looking at ways of trimming this down, and admittedly, there’s a few chapters with low hanging fruit I can get rid of; but I think I’m in need of a lot more than that. My editor was suggesting getting rid of entire main characters that don’t have as much development as others.

But at a lot of turns, it feels like trimming out X causes 5 other problems (plot points lost, throwbacks disconnected) that might threaten to either make the book soulless, not make sense, or even fail to reduce word count when I tie things together.

The option of simply splitting it into 2+ books has been there, but…it doesn’t seem practical. There’s a very clear villain, with a steady buildup to their dethroning, that would feel unsatisfying pushed off to another story.

If I assume publishers, or even just readers, would show only mild interest in a 300k word book, it makes me feel a bit stuck. I’ve already committed a lot of time to the story, and it feels grueling to go back and redo large parts of it; while also aiming to make it shorter.

Curious if anyone has thoughts on what they’d do in this situation.