Quote
It is better if you move your bitcoins first before you go claiming shitcoins with the same private keys. This way your bitcoins are not at risk.
Of course. I always move them first. But they're still within the same xpriv.
The recommended process is to create a completely new seed, and then move the coins to an address on that new seed. The easiest way to achieve the new seed in Electrum is to simply create a new wallet and write down the new mnemonic.
However, your idea of "restoring" a new wallet with your current mnemonic and then adding the extension will work just as well to create a new seed (which gives a new xpriv)... not sure that's the most secure option tho, as you're effectively reducing your security down to however many characters in your extension, should a shitcoin wallet leak your mnemonic (like that "Electrum Plus" scam wallet was doing)
(assuming the attacker knows you are using mnemonic extensions of course 
)