As for a centralized mixer using this - there might be some problems (e.g. liquidity and perhaps even anonymity if they try to grab more liquidity with multiple nodes).
Becoming a taker, I would always provide strong liquidity, forever, which is built up from a wide variety of UTXOs which continually refreshes naturally based on regular mixing, let alone JoinMarket participation.
As far as anonynimity is concerned, I am unsure of what the issue would be? I would need to keep track of which UTXOs had been presented with in combination with others, which is not a huge task, I think the JoinMarket software already does this, however I would be writing the Maker implementation code from scratch (I don't code with Python) and later a Taker implementation for on-demand ad-hoc Cold purposes. I wouldn't have 4 "mixing depths", I would do my own entirely separate thing picking UTXOs and Output Addresses accordingly based on my own requirements.
And I'm still not certain if you're solving a problem that actually needs solving. I use CoinJoin for non-custodial mixing. I use a centralized mixer to get coins faster if I need to and if I'm willing to take the risk. Jumping the TX graph is a non-issue for me after several rounds of CoinJoins and even if it was, a centralized service using CoinJoin has the same problem, doesn't it? If I use it a few times they can't ensure that I won't get my own coins.
It does have the same problem, in terms of making coins produced from the Mixer look like they've been produced from a CoinJoin, which is why this is a discussion only for the moment.
There are key couple of problems it does solve:
- Chain Analysis companies, would find it impossible to keep track of Mixer wallets / clusters. no matter how much they used it, or how much dust attacks then send.
- Coins sent out from the Mixer could never be tainted with past activity, other than endless CoinJoins
- Coins sent to the the Mixer could never be associated with the original user, due to the endless CoinJoins
- Possibly most importantly and selfishly, I may be able to withdraw coins myself, with confidence
