Ok, we have to separate the types of chainslip, possibly by degree. Since it happens only after coasting, my first guess is that there's a lag before the ratchet's pawls re-engage. We usually see this when people use grease on the ratchet. The springs are very light, and grease behind the pawl can act like glue and delay them in the retracted position for a bit. The solution, is pull the freehub off again, clean up the pawl assembly, oil with a sticky oil (like Chain-L, or Phil's) and reassemble being sure the spring is working correctly.

Or the chain may be slipping over a worn sprocket. This is common with new chains and old cassettes, but usually only happen under load, and has a very specific sound and feel. It could also be that you have a burred tooth that the chain hangs on sometimes, or a stiff link, but these would be fairly consistent rather than only after coasting.

Lastly it could be a simple RD trim, or hanger alignment issue, so that's something to check too. Especially as the RD and pulleys were worn.