It’s a good idea to think about these things critically and it sounds as though you are in a good financial position right now. In my experience having some financial independence is the difference between being able to weather a relationship storm or not.
But I’m afraid you can’t futureproof your life entirely or predict what might happen if… your gamble is that you’ll always have your current earning potential and be able to work flexibly and maybe you will.
Although moving somewhere cheaper if you split up might be an option, you also have to consider that once your child(ren) are in school that flexibility diminishes, and that he may well argue that you can’t up and leave with his kids - especially as his job is tied to this location, and you can do yours anywhere. You also might consider how you’d want to co-parent if you split and whether you’d really want to put so much distance between your children and their dad. So moving somewhere cheaper might not be an option.
Running a household by yourself is expensive, it’s not just 50% of your current costs.
If you have one child that’s one ball game, but if you have more than one things become much more challenging logistically and financially.
What if you, or he, become sick and that impacts both your incomes? Suddenly he isn’t earning at all, or has to reduce hours, can’t do picks up/drop offs etc? What happens if you start earning less or you have to jointly pay out more to cover caring/household responsibilities.
As PP have said - marriage offers some protection if you earn less/your career is held back by raising children. It might not protect against everything, but don’t assume you’re invulnerable either, or that it is possible to have all bases covered - life doesn’t usually work that way.