i've spent under $150 on every individual item in my kit so far and it feels silly to blow past that on a mixer.
I wouldn't count on the interface doing everything you dream of, even if it seems like it can based on specs. I owned the allen & heath, but traded it for the mackie one. It's okay, but there's a few annoyances.
Keep in mind that the FX send will also require a return (may sound obvious, but it's not to everyone), therefore, it will consume an input; they basically never have a dedicated return (it's pretty much pointless).
The FX send itself might not be really necessary, but I don't know enough to say for sure. I think they could just label it AUX, and that any auxiliary, or monitor send can work for FX... I'm not sure. I've used them before and the FX to monitor / sample.
I recently bought a Mackie 1604 for a hundred bucks. It's from 1993 and they were basically designed to fall off buildings. Super transparent and likely to outlast this thing due to being much more operable. Not as pretty or quirky, but that doesn't really factor in to my use case.
For stereo compression, you can get a brand new Really Nice Compressor for a couple of hundred bucks. Steve Albini has these in his studio, so that alone should indicate their high quality.
A Focusrite Scarlet 8i6 would serve as a mixer if you don't mind being tied to a computer.
But if you just needed a mixer, the Yamaha MG series stuff is fine and in line with your price point.