Mix happens to yourself. There are plans to extend it to mix to others with arbitrary amounts: https://github.com/nopara73/ZeroLink/issues/75
If you want to send >0.1BTC, then you need a lot of rounds. Again, there are plans to fix this issue: https://github.com/nopara73/ZeroLink/issues/74
However joining together mixed coins after mix is dangerous, while in practice it doesn’t seem to be an issue, due to many people keep remixing coins, but you lost your mathematically proven anonymity guarantee. More on this here: https://www.reddit.com/r/WasabiWallet/comments/9llfua/do_01_inputs_mean_01_outputs_as_well/e77w4hs/
On the other hand joining together more coins than others will likely inevitably result in deanonymization, since a whale cannot hide between small fishes unless he acts like a small fish himself.
Again, refer to the above two ZeroLink issues, there are ways to fix this, and as soon as I’ll get there I’ll be working on it. The thing is, fixed denomination is is the state of the art of Bitcoin privacy, which is not that bad, since the privacy basics are there, mathematically guaranteed privacy is achieved, the only things those need to be improved are the costs and the convenience of mixing (eg. be able to mix under 0.1BTC and make it less dangerous to join coins together post-mix.)
> Will you send the change back to my HD wallet or do I have to mix first 0.1 BTC and then send my tx from there?
Your HD wallet can compromise your privacy. If you send the change back to your HD wallet and it doesn’t have a coin control feature that lets you keep the pre-mix change and the post-mix mixed outputs separated, then that’s pretty much it. Also there are network privacy issues, which unless you are using your HD wallet through Bitcoin Core AND Tor, you are risking getting deanonymized on the network level.
Btw HD wallets are clear targets for example against supply chain attacks, so IMO paper wallets are more secure if you are looking for security and at least they don’t compromise your privacy.