The company I currently work for, Coffee Meets Bagel, does aim to help form serious relationships specifically, and does operate in the US.
The anti-Tinder, till-death-do-us-part market exists, and is likely somehow under-served. It's just a bit harder to market for, and the matching is much harder to implement than Tinder's.
I tried CMB a few years ago and it showed me non-stop Asian women. Nothing against them but it's not like they are the only type I want to date. Has this been addressed?
A huge lot of things has changed since a few years ago, at least on the technical side where I am. The matching engine is now quite different. Consider trying again.
I was using CMB until recently, and can confirm carabiner's comment. Asian women seem to dominate it to such an extent that I have friends who use the app specifically because they prefer Asian women!