So can I turn this watch-only wallet subsequently (=now) into a full wallet?
Simply by importing seed or private key?
Or do I need a full new installation.
Simply create a new wallet with your seed phrase or your private key.
Later I want to toggle this wallet occasionally from watch-only to full and back.
Is this possible without fiddling with seed and/or private keys?
You need to have two different wallets. One watch-only with your address and and also a wallet created with your seed phrase (or your private key).
May I ask what do you want to achieve?
If your purpose is to increase your security, you should create the wallet contaning your seed phrase (or your private key) on an offline device (preferably to be air-gapped).