What do you mean by spreading out UTXOs?
A SegWit output is as secure against QC with or without CoinJoin'ing.
Addresses should not be reused anyway. So until spent, the PubKey is not public and thu...
That's a great point about SegWit and QC security, but CoinJoin still offers additional privacy benefits and protection against analysis of spent coins. And you're right, addresses shouldn't be reused. Glad to see BTC pushing up.