First time repairing a rear wheel hub, I am hearing bearing on bearing rattle/clicks when spinning
When spinning the wheel before putting it on the bike I was feeling a bit of crunchiness but no noise.

My guess is that the crunchiness you felt has become the noise you hear with the wheel bolted into the frame. Crunchy is always bad when it comes to bearings.
Sadly, you need to tear it back apart and find the crunch.
The comments have good suggestions and I'll add a few:

MaplePanda says - Is everything nice and clean? Sand, dirt, metal bits, etc. are possible causes.
Carel says - Correct number of bearings?
You replaced the bearings, axle and dust caps. Possible wear on the cup or cone bearing surface?
Could the dust caps be set a little too deep coming in contact with the bearings?
When you say &quot;The wheel has a tiny bit of play&quot; does that mean in the bearing adjustment? If not make sure the cones are not too tight.