Thimble0548 from my knowledge, you can only really use the default SMS app on user profiles. In theory you could do just that and deactivate this app for the owner and all other user profiles, but this is highly recommended against since it's a system app and deactivating it can break a lot of things down the road and you likely won't be able to identify the problem. So don't do that!
First and since you seem to worry about receiving malicious silent SMS, check out the FAQ section about this topic. You might be happy to know that it's not a big threat by itself and especially on GrapheneOS. But if you want to fully protect against it, airplane mode is the only way.
Now my suggestion is that you get yourself a mobile router like the GL.iNet MUDI 2 and put your SIM card there while your GrapheneOS device remains in airplane mode. This way you detain the SIM and SMS reception but keep internet on the road through WiFi. It won't protect you against triangulation of course, but that doesn't seem to be an issue for you. For receiving SMS, you'd have to out the SIM card back into a phone... maybe a dumb phone or dedicated phone exclusively for that task?
Before you do all that, I highly recommend to take some time to make a threat model for yourself. Maybe all of this is not necessary or you're hardening the wrong thing while leaking your data elsewhere.
Also, I'm not speaking for the project and you shouldn't trust a random stranger on the internet, so better wait to see at least some more opinions from the experts.