I want to be able to unlock my private keys for a specified amount of time by typing the passphrase. The same as Bitcoin Core does. I spend from Mycelium maybe once a month. I can handle typing my passphrase that often. I type it into Bitcoin Core more often than that, and it's not a problem at all.
Use BIP38 encrypted paper wallet, which you can spend from using cold storage option in Mycelium. Change goes back to the paper wallet address.
Address reuse? But I want an HD wallet, not an old-style address-reuse wallet.
we keep having to explain that this is terrible for privacy
Quite!