I just recently switched to GrapheneOS from LineageOS and ran into this same thing. I am also a Proton user and have Proton Drive. Here is what I did to get contacts, sms messages, call history onto my new GrapheneOS device without using a computer or other complicated means.
On my old phone I downloaded this app
https://f-droid.org/en/packages/com.github.tmo1.sms_ie/
I then clicked on "export messages" to my Proton drive and uploaded my messages.zip file
I clicked on "export Call logs" and uploaded my calls.json file
I clicked on "export contacts" and uploaded my contacts.json file.
I confirmed that all 3 files were uploaded onto my Proton Drive.
On my New to me Pixel 7Pro phone I downloaded the Fdroid store and downloaded the exact same app:
https://f-droid.org/en/packages/com.github.tmo1.sms_ie/
I then opened the app, clicked on "Import messages", you'll run into a message that says "you must allow sms import/export" to be the default SMS app. Click on allow then In the upper left hand corner of the App I clicked on the hamburger menu, selected my Proton Drive, found my messages.zip file and clicked on it and the app automatically imported the messages.
I then went to the native Graphene OS app and made it the default SMS app and the phone automatically brought over the SMS history!
I then used the sms import/export app to import my call history and contacts via Proton drive and my new Graphene device reflected my old Lineage device.
The great thing about doing this method is the encryption in using Proton drive versus a Google or other non-encrypted service.
As a bonus to users of Signal.
On my old device I navigated to the "account" section and clicked on "your account data" and I exported my Signal Data as a JSON file to my Proton Drive.
When I set up my Signal on my new device, I used the signal.backup file to bring in my signal history to my new Graphene device. I had to use this method since I could NOT get the android "device to device" transfer to work..
Hope this helps.