Anything to worry about if you use mixers and not doing anything illegal?
Yes, as there is currently a hunt ongoing for mixers, and for users of mixers. It depends more and less so based on if the mixer has been seized or proven to have illegitimate involvement, but nonetheless, I would be taking precautions, especially if you are using centralized services.
Will any one come knocking on your door if you use mixers and not doing anything illegal?
Probably not. Though the thing to be worried about is being asked for KYC on virtually any centralized service. As users said, that's fine, just use decentralized and p2p services instead.
Will posts like this be banned too after Jan?
No, as others said - no promotion, no link sharing = OK.
Will using mixers be an expensive?
Maybe it will create temporary competition, maybe costs will go up, maybe they'll stay the same. Technically, less spent on marketing means savings for the company, so I don't see why costs would go up.