5) Here it's about scaling what I already have.

I have Trezor, backuped with great care and not-small investment (Shamir, stamped on stainless steel, very carefully distributed to match my threat model)

It is ideal device to use as the universal key.

I can do some other solution, I can buy Ubikey or something. But it just adds complexity and cost to achieve what I already have with Trezor.

Trezor is perfect match for this usecase.