I have an advent 9 system built into a bike of mine. Currently it is out of commission but I’ll be fixing it up soon.

the advent 9 was advertised as a single chainring system but I installed it with a double crank set. The double is too high and I end up spending a lot of time in the lower 34t cog.

im thinking of doing it the “proper” way and installing like a single 38t instead.

so the issue I’m wondering about is chain angle. I believe that 9 speed chains are not designed for angled use. With a single chainring there will be more extreme angles. Extreme angles increase chain wear and decrease efficiency

rear cassette goes from 12t to 42 t.

Hmmm. My 9 speed Advent 1X came with a 9 speed chain. However, I replaced it with a 10 speed chain that also works well. If you're concerned - and I don't think you need to be - just put a 10 speed chain on it.

I have no idea of 9sp chains are not designed to be used at an angle, bit I have put Advent 9 on 2 bikes and it works fine.
It'd be a pretty cruel joke to design drivetrain components to be used with a chain that can't handle the rest of the drivetrain.