You can already choose “Prefer LTE” for the data SIM and “2G only” for the other (voicecall/messaging) SIM. That’s pretty much what you want, right? At the moment you can’t lock data SIM at LTE (e.g. prevent switching technologies) but that may become possible in not so distant future, provided that your modem supports it.
And unfortunately I don’t see “LTE and GSM only” option in the low-level modem API:
https://android.googlesource.com/platform/hardware/interfaces/+/e4f5f02/radio/1.0/types.hal#651
Looks like you can’t ignore WCDMA (3G) without ignoring either LTE or GSM (2G) along with it. I have no idea why.