If you do end up using a Mixer and they hand you out Private Keys, make sure you are using them ASAP. I would not hold my Coins in their generated Private Keys for too long. This is just to lessen the likelihood of falling as a victim of a Scam, if it ever happens.
As far as I have used mixers, it has rarely happened that a mixer offers the possibility of keeping assets with them. The only one I remember that had such an option was WWM, which offered a staking option. Although it ended badly.
But this is not because they are Mixers. It is an advice I give to everybody even regarding Centralized Exchanges that are well known and legitimate. It is just an advice for the Safety of your own Coins. Custodial means you are not really the owner of your Money. Be the owner of your Money.
This should always be repeated as a mantra, for all services. Even mixers
legit are the safest from this aspect.