Talb0
GrapheneOS fully supports Wi-Fi Calling (and texting). No need to connect to cell towers; just a Wi-Fi with a proper internet connection will do. The only caveat is that your carrier has to support Wi-Fi Calling; if it doesn't, there's really nothing GrapheneOS can do. I won't get those bonus points as I don't know about Mint Mobile; sorry. Maybe their website has info about it? Usually carriers tend to publish their capabilities.
Wi-Fi Calling can be enabled per SIM in the settings. Note that it's off by default and needs to be manually enabled (only once though).
Settings > Network & internet > SIMs > [Your SIM here] > Wi-Fi Calling > Use Wi-Fi calling
After enabling Wi-Fi Calling, Wi-Fi texting is enabled automatically as well; they're both controlled by the same Wi-Fi Calling toggle. Do note that if you use multiple user profiles, toggling Wi-Fi Calling on/off can only be done in the Owner profile, and the change will affect secondary profiles as well system-wide. Wi-Fi Calling works without issue in secondary user profiles as well once toggled on.
I can personally vouch for this. While my situation isn't a one-to-one with yours, I'd say it's somewhat similar. I use a Pixel 9a as my daily driver and have Wi-Fi Calling enabled. I keep my phone in Airplane mode nearly all the time and thus unintentionally simulate having no actual cellular coverage; no cellular connection means no cellular connection, whether it's due to Airplane mode or actually being outside of cellular coverage. My text and calls work just fine with Wi-Fi Calling enabled and yours should do just as well (provided that your carrier supports Wi-Fi Calling).