Trezor now got an upgrade. You can upgrade your Shamir.
- Get Trezor T or Safe 3/5
- Create new wallet (you have to create new seed, it won't work with old backups for technical reasons)
- You can create any Shamir setup (3/5)
- You can later generate **additional** backup (for example 1/1, aka Single Seed) if you need to.
https://trezor.io/learn/a/upgrading-to-multi-share-backup