This is the uncertainty I'm talking about. The user does not know beforehand how much money will be forfeited because the decomposer treats the remainder as dust. Is there a page that explains how this works behind the scenes, ensuring users aren't surprised by paying significantly more than expected?
Yes -
https://github.com/WalletWasabi/WasabiDoc/pull/1859It's under 'Coinjoin' tab within 'Wallet settings'.
I had found that, but I assume that allows me to send only to Wasabi wallets, correct? I cannot send the joined coins to Sparrow or to a bunch of addresses I select?
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Yes, the feature only allows you to move coins from one of your wallets to another wallet you have loaded on the same client.
If you want to send to selected addresses, you can do so through
Wasabi's RPC or with BTCPay's coinjoin plugin (via the 'schedule payment' workflow).