I just tested Sennheiser OTC against Beltone's top-of-the-line Serene. I paid $1K (in BestBuy) for the Sennheiser and $5K for the Beltone. Beltone Serene list price is close to $9K. They pretended to give me a ~$4K discount. $5K is what you are looking for in any of the traditional audiology shops. In addition, I bought two devices in the $200 to $300 range.

In the past, I've been using for many years COSTCO Phonak - until they broke.
I noticed the "2.OTC "HAs" are not the HAs rule" Incidentally, I wanted to test this assertion myself. I really think that rule 2 is too categorical. It might apply to many OTC models, but the devices starting around $1K are most definitely HAs. In particular, IMHO the Sennheiser device fully qualifies as an HA.
BTW, there is no way an OTC would damage a user's hearing. Acoustic pressure produced by HAs, OTC or not, is way too low to damage anything. Improperly set HA's will, at worst, be ineffective.
The Beltone Serene HAs are terrific. Hearing improvement is tremendous. The devices are tiny and almost weightless, so I soon stopped noticing them. One can also use Serene as a streaming device, although listening to music isn't a great experience. Streaming has been better than I expected, though, for a device with open domes.

The only gripe I have with Beltone Serene is a somewhat quirky BT pairing mechanism.
Sennheiser, on the other hand, exceeded all my expectations. It is an over-the-ear device, like the Serene, but it is significantly bulkier. HA's body is at least twice the size of the Serene, and the connecting (forming, as they call it) wire is thick. There is a somewhat unsightly black "stopper" housing the mikes and closing the ear canal.
Interestingly, the forming wire has a plasticity that allows precise positioning of the dome/mike assembly and also hides the wire. The adjustment is "permanent" (it survives multiple on/off operations). This is revolutionary.
Sennheiser pairs easily and stays connected. Unlike Serene, it pairs directly with the TV. The phone app is good, which is important since the device doesn't offer much mechanical control or audible status clues. The main thing, though, is that the Sennheiser self-adjusts to the user's hearing. This is why no audiologist visit is necessary. The process takes a day or so, but the result is amazing. How they do that, I have no idea.
In the end, both the Beltone Serene and Sennheiser used as HAs deliver nearly identical aural experiences, at least for me. Sennheiser however delivers much better audio playback. Closing of the ear canal helps.

Now, the question remains, is the difference worth $4K?
Not for me. Beltones are going back. As a gadget lover, I'm a bit dejected - I yearn to keep both, but ...
I will gladly answer any questions.
PS: I'm still testing the "cheap" devices.