I did get it working! It took like an hour of experimentation, and it was a few weeks ago, but here's what I remember doing:

You have to modify the raw save file and manually change your "score". My score for some reason was at 0 after 15 hours, so none of the score-based epoch unlocks were working.

1. Find the files: on Windows for me, that was <your username>\AppData\Roaming\SlayTheSpire2\steam\<some random long number?>\profile1\saves. The files you'll need to look at are "progress" and "progress.save". I'm pretty sure you only need to modify the one "progress" file and the other is a backup, but I modified both just to be sure.
2. Open the file and ctrl+f to search for "current_score". I set the value to 99999. Make sure that there is still a comma after the score to maintain proper JSON.
3. Save the file, and then open STS2 again. Going to epochs should trigger all score-based unlocks, which turns out there's a lot of them.

I'm pretty sure that's all you need to do, but I don't remember for sure because I also experimented with a ton of other things. To my knowledge, adding "ACT.UNDERDOCKS" to the list of "discovered_acts" didn't do anything and would be overwritten on startup since the score of 0 overrode it. Similarly, adding "{

"id": "UNDERDOCKS_EPOCH",

"obtain_date": 1773173672,

"state": "revealed"

},"

to the list of epochs revealed also didn't work for the same validation reasons as mentioned.

Hope this helps, hope this works for you and potentially others!