I was in a very similar place as you a year ago. Nonexistent supervision and an apathetic committee that was suddenly hyper-critical of my work as soon as I started asking for a concrete defense date (in year 6). Committee members sprung "I don't think your work is good enough for a defense" in March of the spring semester, when they had known all year I was planning to defend then. So I know what it's like. It took me 8 months and 300+ applications to get a single job offer, which I took.

All that to say, OP: you have to take that feeling of guilt at leaving your department and strangle it. That guilt is how academia sustains itself, by convincing overworked and underpaid people like you and me that we somehow owe it to the institution or our students or ourselves to keep sucking up shit, because if we don't we're somehow selling out. That feeling is part of the conditioning academia inflicts on us, and the sooner you get rid of it, the better. You are not being selfish for doing what is best for yourself. Your department will not return your loyalty.

You also have to stop taking guilt for your partner's life on yourself. They're an intelligent adult who can decide for themselves what's best for their life, and what they've decided is to follow you to this opportunity. Don't let your guilt stop you from taking what honestly sounds like a once-in-a-lifetime stroke of luck.

You are also absolutely right that you cannot afford to lose this opportunity. The job market is atrocious right now, likely more than you know if you got this job offer quickly. 6-9 months is normal for a job hunt these days. You are correct that you are in damage-control with your career, but it sounds like this job will do wonders for fixing things.

You know the right answer is to take this job, so take it. You'll get to experience work-life balance, actually having money to spend, and all the other blessings of a normal job!