I might be missing something, but isn't part of the allure of it all being a blockchain the ledger that will, for ever after show that such and such wallet received 30k on a particular date & time - potentially requiring some stitching that would probably be trivial to do if the actual transfer was broken up into a series of smaller transfers?

And if you're using a shared wallet that obscures internal money movements then the feds can still see any entry/exit point activity - they might just be uncertain about what happened to the money in the middle.

I guess if you created a private coin for your nefarious dealings you could keep it really hidden (maybe one tied as a stable coin to a more major coin) - but only so long as you managed to keep the feds from getting a ledger view of the private coin - which would require limiting the spending power of that coin to only parties you strongly trusted which, again, would strongly devalue the coin.

The real world equivalent might be deciding, as a criminal ring, to use gems as a currency to avoid handling USD directly too often - but those gems become worthless apart from their inherent value as soon as trust collapses.