Comment by onelastjob

Yes, exactly this. As long as the fiat on-ramps and off-ramps are KYC'ed everything that happens in between is an open book. The individual can just use cryptocurrency tax reporting software to provide a trade history for their taxes, and if they don't do that then the IRS can easily track all of their transactions since the IRS will know what addresses they are sending to from exchanges. There's no need for every DEX (or other service) along the way to KYC their users.