Thank you for your reply! I really enjoyed reading it, especially because it gave me the awareness that I'm not the only one having this issue.
To give us a bit of hope: I've seen some channels within my niche, that have recovered from this. At least I assume, they have - based on their channel stats & views and how they evolved over time. You can follow a channel's growth & evolution on Socialblade. For instance, two of my key competitors had this problem, too: new channel growing fast; first 7-10 videos with a few thousand views each within days. Then, somewhere after the 10th upload, the views dropped dramatically. The next 15-20 videos had no more than a few hundred views each. One competitor stopped uploading after his 25th video or so, discouraged, probably assuming his channel is irreversibly damaged and that it's not worth the time & effort. The other one though kept uploading and then, out of nowhere, the views for one of his videos (somewhere between the 25th and 30th upload) literally exploded, generating 150k views within a week (which can be considered viral in this small, targeted niche). After that second blow up, that guy's views didn't drop again. That second boost in views occured somewhere between the 3th and 4th month of uploading, judging by the upload frequency (3-4 videos a week on average).
I don't want to spread false hope. Maybe that guy did some cross promo to give his content an extra push manually. Maybe he was lucky. Who knows? But at least I can say that his channel was or had been in a similar situation to ours. So, from my point of view it is definitely worth "playing the waiting game", as you nicely described it - for at least one more month of consistent uploading HQ content, even if I currently don't have the motivation to do so - just to see, if this can be overcome and fixed or if this is permanent.
Fingers crossed.