It’s is really, really hard OP. And I think they people who don’t work in classroom-based school teaching do not always understand the time, effort and emotional labour that involves. I’m not a teacher now, but that was the hardest time of my working life (and I was young with no kids!)
As for keeping a marriage thriving, I do know lots of happily married couples with 2 great careers, and here’s what I’ve noticed:
They chuck money at the problem. Outsource everything you can afford to, but especially cleaning.
No guilt about taking time off to spend together, or of spending money doing lovely stuff together. We have a weekend away each summer, and take a few days leave in the year to just have lunch, a walk, go Christmas shopping in London. Our kids are a bit older and every Sunday we get up at 8 and walk the dogs together.
They give each other time to follow their own interests and hobbies. Not a weekend cycling around the Hertfordshire countryside for one and a weekend of homework and laundry for the other.
They avoid the ‘I’m more tired than you’ game (although I did instigate a game last night, to my shame!). They are a team and look out for each other. They argue in private and don’t bitch about each other.
They both oversee the traditional ‘wife work,’ as and when time allows. They look out for each other when times are tough, support goes both ways depending where it’s needed at that time.
Holidays are sacred, a reward for months of hard work and great fun, whether it’s in a tent or a 5* resort.
Money helps, outside support helps but both being on the same page and fundamentally liking each other really matters.