I believe it'd be easier when they already have a certified phone from a given brand, because then the phone's radio has the appropriate XML settings already in the modem configuration. I didn't have VoLTE on my OnePlus Nord because Plus explicitly disabled that feature in Oneplus' radios, forcing me to swap XML files in QPST, to "claw it back" (phone supported, but modem config said it didn't) something I thankfully didn't need to do with my Nothing.
Then again, 5G slicing config is a different beast altogether, so if they didn't provide the settings for Nothing in the ROM itself, there's really nothing we can do, except ask both Plus and Nothing to change that, which I'm not holding my breath for tbh.
I'm moving to T-Mobile as soon as my plan ends.
Please note, I go purely on my three hours of calls with Plus's tech support and my prior experience with modding my Nord, most of this is speculation.