The shamir gave the option to have just a single share? Have you tried it and it worked? At least you suppose to have 2 or more shares. If it is a single share, then what is the point of having shimar as a backup plan.
If you do not need shamir purpose of backup, then just go for normal seed phrase backup directly. Go for segwit derivation path instead and not legacy. With segwit, you can save money on fees while making transactions.
Yes, the single share is actually the default backup option. I tried it out last night. After backing up the wallet, I reformatted the Trezor and tried restoring it from the 20 word list and it worked. I guess I'm not understanding the advantage of this over a regular 12 or 24 word backup.