This is a very valuable information, thanks for sharing.
I have to correct one error in your description:
‘For single wallets, a seed backup is ok
For a multisig wallet, a full backup is necessary’
The type of backup has nothing to do with single or multi-sig wallets. The seed backup can only recover Bytes and other (non-private) assets which are publicly visible on the DAG-chain. All other information, like smart contracts or Blackbytes, can only be restored by a full wallet! And keep in mind that you should create another backup every time you create a new smart contract or receive more Blackbytes.
Look into @papabyte’s reply how a multi-device setup can be a convenient replacement for a full backup.