40t x 9 is slightly lower than 50x11 and slightly higher than 50x12.

Its high for gravel but depending on conditions it’s not excessive.

Anyway, I’d be checking derailleur hanger alignment, the derailleur, cables, chain, limit screws, tension, and gunk all before worrying about a new cassette.

Are you skipping in both the 40 and 26? It could be a chain line issue too.

All cassettes are eventually wear items. If it’s not today, it may be next year.

I’d be uncomfortable with the “last in the world” cassette. You need to make a plan for when that happens. Possibly eBay, but more likely moving to a different cassette.