[Google Pixel 2 XL ROMs, Kernels, Recoveries, & Oth] GROOVYJOSHCLARK:
I get that and I am familiar with it even though I do not use it myself. What I am saying is you will get far better results asking in the seedvault areas, than in the ROM forum section when its 3rd party and changes the core functionality of said ROM. Its possible someone here uses it and can assist you with the steps, but keep in mind, its 3rd party and thus isnt top of the concern for most devs. I am only mentioning this to get you the best assistance possible but not to frown on the idea, go for it if you would like to try it. Report back your results, or if you get it working so others benefit.

I for one, only use Google for Google backups (TXT/contacts as well but no MMS) with GAPPs (OpenGapps) for my framework so I do not use seedvault myself. I rely on SMS Backup and Restore Pro (once Carbonite) for all SMS/MMS backups locally saved locally or cloud (XML file). In that 3GB+ XML file, it has every single TXT/MMS/PIC/VIDEO I want saved for transferring to another ROM. In this one single file, contains the entire message, plus attachments, plus conversation (so everything), it saves local, or Google Drive, dropbox, ETC. (or both). I use TiB for all app related data (game progress/configs/ETC). SMS Backup has the same version with ads thats free, and it is my GO-TO text app as it backs up everything within the message. You choose the date range (or everything), what to include (pics/attachments, or not), full backup, scheduled backup, incremental backup, the options are limitless with it and it works everytime, without failure (I have used it for years and flash ROMs all the time). I have it backup nightly so I never lose a TXT or MMS message!

It all comes down to what you're trying to accomplish. If removing Google's servers from the mix for security of your data, think again. You're now saving to an open source system that you personally cannot verify the safety yourself. This means your trusting open source over Google for security when they (seedvault) have much less to lose if they're hacked, but Google has EVERYTHING to lose (if they're hacked). If Google is hacked, there goes their security reputation and sales will show it, so they're very security oriented (they're not perfect, no system is) but they have them most to lose so (for the time being), they're who I'm choosing. This is way I prefer to keep my backups and data local (TiB/SMS backup), both are saved local to multiple locations so it's safely housed on-prem and without cloud infrastructure. Everything else is either freshly installed / configured, or pulled down from Google on setup, then back filled with local data (TiB/SMS backup). I'm not saying Google can't be or won't ever be hacked, and I'm also not saying that they're not safe themselves (seedvault) being open source (hell almost everything I use/write is open source), but in this aspect, I trust Google over them (seedvault). I've used seedvault in testing and found nothing fundamentally wrong, or no reason not to use them, but all I'm saying is to rethink it through what you're trying to accomplish if security is the driving force to use seedvault.