a) why did you include 0.05001234BTC in the first please i am confused
To demonstrate that Wasabi CoinJoin have fixed output of mixed coins and usually there's "left over" coin which isn't mixed since it's amount is too low.
b) if mixing is not about using all of mixed coins in a single transaction then how is it done probably a.k.a. post-mix i mean
There's no single/best how it's done probably, but don't spend all mixed coin and following guide by wasabi which i mentioned on previous posts should be good enough.