I know this discussion has been discussed in a similar post before, but things are changing.
For a long time, Google whitelisted carriers for Pixel, meaning that many carriers with fully compatible hardware couldn't use 5G and/or VoLTE/VoNR on the Pixel (this was primarily limited to certain carriers in Europe and mainland China). For a while, Pixel IMS offered an alternative, allowing VoLTE to be enabled via ADB+Shizuku (However, 5G was still only enabled with root access). While ADB isn't secure, it was a viable workaround, given that the changes weren't overwritten after a reboot or disabling ADB.
However, in the latest Google October security update, Pixel IMS was specifically blocked and no longer works. Given that 2G networks have been deprecated in many regions and have proven to be insecure, perhaps reintroducing the Carrier Config override option is now a viable option.
Aside: I understand that GrapheneOS developers strive to stay as consistent as possible with AOSP. I respect and appreciate their work, and I agree that this was reasonable for a long time. However, as we're seeing now, Google is introducing more and more restrictions in AOSP, including but not limited to targeted blocking and delayed open source security patches. This is clearly increasingly conflicting with GrapheneOS's security goals. If Google turns GrapheneOS into an operating system that can't make calls in many regions (due to the lack of VoLTE and VoNR), then we're clearly being slowly killed by Google.
There seems to be an implementation of this feature on Github, so merging it might not require much effort. It includes the option enabling VoNR and VoLTE, and the option to choose between 5G SA and NSA.