The Third Efforts II
As regards the issue of the huge latency when I use the Aliexpress DCDZ HDMI-CSI adapter + GoPro Hero5 black, I was suggested in HDMI-CSI converter with GoPro Hero5, huge latency, what to do? that I should reduce the value of the parameter BITRATE_PERCENT (many thx for the help 
). It is not totally clear to me if that value affects only the air or both air and ground, so I updated it through forced smartsync.
I found that I had to reduce that value from the default quite substantially. After some substantial testing, I finally ended up with using 27. The bit rate is then on the order of 5 Mbps, which is much lower than what I observe with the 10Eur cam and the default BITRATE_PERCENT value, but I somehow got more disturbances when I went higher. So, 27. LOL.
Anyway: Aliexpress DCDZ HDMI-CSI adapter + GoPro Hero5 black works
BTW: In case anyone wonders: As regards using the DCDZ HDMI-CSI adapter with RPi one can find lots of web pages which tell that one has to do this and that patch of the kernel or of a driver and what ever fancy thing. The latest I found was saying that e.g. raspivid would not support it, and that raspivid would now spit out a text warning message. However, I’ve just installed a fresh latest raspbian image, and with it it works just all fine out of the box. So, it seems that this issue has been finally resolved.
The Fourth Efforts
Having been curious about how the RPi Zero would do, I realized that while I can’t yet connect my 10Eur cam to it I can do so for the DCDZ HDMI-CSI adapter + GoPro Hero5 black (the DCDZ HDMI-CSI adapter comes with the required cables). So, I did that. And:
It worked out of the box. Fantastic.
Now, given the above cpu numbers, I was most curious about how that is with the RPi0. And, well, it can go momentarily to high values, but most of the time it is in the range of 25-30%. This is fantastic, isn’t it?
It’s also somewhat confusing. Just a moment ago I have read the release notes for OpenHD 2.0.1, which for the RPi0 says that “CPU use should now be around 25-30%, down from 60-70% or higher, The fix only works on Stretch at the moment, so you MUST use the Stretch image on the Pi Zero for now”
Remember, I’m using OpenHD 2.0.0 buster, and that’s also what I used for the RPi0 test, and I get miracuously similar values of 25-30%, on buster, without that fix…
Anyway, I guess the great message is that the cpu load on the RPi0 is, or is going to come down to the expected level.
Cheers, Olli