Your question is actually trickier than it first seems like, so I'll need to answer it thrice.
[1] The obvious answer to "I want to compress my videos as much as possible with minimal quality loss" would be "use MP4 with h.265/AAC or WebM with VP9/Opus".
[2] However, that answer isn't necessarily applicable here as you explicitly don't care about video quality.
Instead, you can use literally any container/codec and set the bitrate to the lowest possible value as bitrate is the main thing blowing up files. Since you want to maintain audio, the overhead the container should be minimal; Matroska has <1% overhead on 64Kbit/s audio and you likely want a higher bitrate than that.
Speaking of audio bitrate: YouTube uses 128 kbit/s AAC that they encode with the "best-of-breed encoder" as their studies have shown that this is indistinguishable from even 320 Kbit/s AAC. Since you don't have access to their encoder, you want to be above that 128 kbit/s target. 160 or 192 kbit/s should suffice.
Seems like we're pretty much done, right? Use any videocodec/container at any resolution and frame rate, just chose the lowest-possible video bitrate that YouTube doesn't reject and chose either 160 kbit/s or 192 kbit/s for the audio and you can upload all your thousands of hours of music.
[3] There is one more problem though: YouTube limits you to uploading 50 videos a day (after that limit is hit, you can upload one more video every 15 minutes or so, until it just blocks you at 100 videos). Depending on your upload speed, you may not actually need to go as far down with video bitrate as possible.
And as final, non-technical thoughts: 24/7 music live streams have become somewhat popular as of recently, you may want to try that instead of uploading. YouTube has a guide on how to make successful music channels
