New devices in the US that have an unlockable bootloader and custom ROM support?
One Plus is the newer exclusion in the list of reducing number of new devices in the US which can run Custom ROMs. Right now, I only really know of the Pixel and the Nothing line of mobiles which can run Custom ROMs.
Xiaomi phones don't support the all bands used by US carriers. Yes the phone will still work for 4G, but you'll potentially be out of service in certain places that require the US-specific bands.
I bought a Xiaomi A3 about four years ago. I'm still using it now but my experience has been mixed.
Two years in, my carrier told me I had to switch phones because it didn't support a bunch of technology they now require. It in fact supported everything including all the correct GSM bands, but ATT was now operating on a whitelist to which my phone did not belong, and they would not produce an explanation as to why. I was able to switch to T-Mobile at a reduced rate, but I find the coverage leaves much to be desired by comparison.
Just don't get xiaomi if you're in the US. I used to have one when I moved from India last year and majority of the bands were unsupported, so I was stuck on 2G-3G speeds.