u/statoshi
Is there anything wrong with using an already active single sig seed phrase generated with dice?
Nope, that's fine to keep using.
Do you still keep the descriptor file on your PC?
Yes, doing so is no different than having a watch-only wallet on your PC.
How did you stamp the xpub/descriptor onto metal, like you would the seed phrase?
I don't go this far but the only user friendly way to do so that I've come across is via a SeedHammer.
xpub should be fine because the descriptor should include the redeem script type inside it. worst case scenario you can always convert an xpub to another *pub type with a tool like https://jlopp.github.io/xpub-converter/
I'd save it in a variety of ways and places, both digital and analog. It's not a security issue if the descriptor falls into the wrong hands, just a privacy issue. No problem if you print out a bunch of copies too and keep them in different locations. Can even print them in both text and QR code formats.
I've tested 70+ metal backups personally: https://jlopp.github.io/metal-bitcoin-storage-reviews/
I'd suggest you read my final takeaways here: https://blog.lopp.net/a-treatise-on-bitcoin-seed-backup-device-design/
KISS: Keep It Simple, Stupid
Reputable recovery service providers listed here: https://www.lopp.net/bitcoin-information/recommended-wallets.html#recoveryservices
