So I'm sure I understand, with my current Electrum wallet I should disable change addresses if I want to make sure my single backup will always restore everything?
No
You imported your private keys, you don't have a HD wallet, no new private keys will be made by your wallet, so if your wallet picks a change address, it'll be an address that comes from an imported key.
You can make a backup every time you import new private keys, that should do it.
Actually, i'm not even sure that electrum will even use change addresses on a non-HD wallet.