Update: I thoroughly cleaned out the cage in the backyard with a hose, after transferring Loki to a small budgie cage for an hour and a half. He was quite pissed off about the whole process, as you'd expect, but is keenly inspecting the new rearranged interior of his big cage after I coaxed him back in.

I still feel rotten about how bad it got, and how Thor died because I let it get that bad. There were significant layers of old shit mixed with seed husks in the tray under the cage, and it would have been harbouring bacteria, mold, etc, and drifting through the air, increasing infection risk for those poor birds.

I have extensive mental health issues, and they cause me to neglect large parts of my life, but as others here have rightly pointed out, that does not in any way justify the neglect of someone else whose welfare is my responsibility.

I've cleaned the cage properly, and also rearranged things to make it easier to maintain that cleanliness. There's butcher's paper lining the tray now, easy to remove and replace every week. I have moved the water bath, the seed tray, and the water bottle around so that they are less likely to get fouled, but still easy to access and clean.

I don't have a pic of Loki in the new cage, since I've given him some quiet time to recover from the stress of moving back and forth, but here is one of him in the budgie cage, looking like some sort of giant.