Tried computer PSU's and that failed miserably. I too thought I had found a great power supply basis.
I built a project a while back, based on a 700W computer PSU (New one).
I had horrific issues with noise on the MP3 part of the project. Just could not cure it.
After literally weeks of filtering, ferrite coils, rewiring and adjustments.... I binned the computer PSU and fixed the issue overnight.
Never tried one since
As for the 7805's....
100uf and 0.1uf caps on the in and the out. Mounted on a heatsink.
I measured the load of the WS2812 Leds to be considerably more than you do. I powered a strip directly off the bench PSU and some of them were peaking at 1.2A.
Certainly more than 0.466A per strip. Not sure in fact why I am not pulling more amperage on the final 12v supply.
I determined they were failing because I didn't have the issue off the bench supply, but they didn't work correctly off the 7805's. Plus... the heatsinks were bulky anyway.
I realise I am stabbing in the dark a bit, and a lot can be down to wiring.
I am super neat with my wiring, and I usually run everything back to one point and star the supplies from that.
So it's annoying when you have to start un-stitching your neat work to troubleshoot