AFAIK with sovryn you are swapping BTC for some random token called SOV, and they put that into some smart contract. Doesn't seem amazing to me.
AFAIK with mintlayer you are swapping BTC for some random token called MLT, and they put that into some smart contract. Doesn't seem amazing to me.
You can not keep your Bitcoin or any coin in an airgapped wallet and earn interests. If you want to earn interest, your coins must have some sort of roles and you have to accept risk and interest you get in return.
If you want something totally safe, ok it is airgapped wallet, and forget about interest.