OK, so I found some stuff at Modland. Seems they have a bit of Oktalyzer stuff at least. Including a couple of hundred modules attributed to FBY (
https://modarchive.org/index.php?request=view_profile&query=69605). I see he also has a website (fby.develer.com/fby2/mp3/index.html) with a load of music in various formats including Fasttracker II, Protracker, Oktalyzer and Sonix (old non-tracker Amiga music program).
Modland also had a folder for Startrekker FLT8 modules, which contained only 1 single module. Guess that format never really took off. There are some modules there with synthetic instruments, though. This is another thing that never really took off. Same thing with MED/OctaMED, apart from a couple of example songs by Teijo Kinnunnen and some nice songs by Dr Awesome, and the awesome Alien Breed title tune by Allister Brimble, I don't think I've heard many modules using these. But all in all, looks like synthetic instruments became a niche thing when we got samples.
There's also a lot of OctaMED stuff, some of it is probably multichannel, but it's not sorted from the 4-channel stuff, so going through it will probably take some time and effort.
Re your comment about being overwhelmed by more than 4 channels, I don't think this is the case, since there are plenty of examples of people going to great pains to use two Amiga simultaneously to get 8 hardware mixed channels. The mentioned FBY site has modules designed to be played together. I believe there is a special version of Protracker that supports syncing two instances running on separate Amigas using a serial cable. And I also believe Urban Shakedown (and probably others) back in the day used two Amigas with OctaMED in 4-channel mode that they manually synced up with a click-track to get 8 hardware channels.
I'm guessing the absence of 8-channel stuff on Amiga is more due to technical limitations and quality implications of doing software mixing of 8-bit samples on a 7 MHz CPU. Going through the Oktalyzer stuff I do notice that a lot of them only uses 4 or 6 channels. And I also remember when I experimented with OctaMED that 6-chan was a good compromise, giving 2 hardware channels and 4 software mixed channels. Also, the added CPU requirements of software mixing made such productions unsuitable for games and demos, at least until 68020 CPUs became mainstream with the Amiga 1200.