I found a nifty way to do a couple things with regards to Channels & an external player such as Plex, Jellyfin, Emby, etc.
I wanted a way to separate channel playlist & epg by source. I have multiple TVE sources and the xml output puts all TVE channels in it, regardless of source. I had a lot of duplicate channels that I kind of wanted to keep, but can't seem to get the stack duplicates to work with any external players like it would with the Channels app. So, I decided that eliminating duplicates was fine and went on to this. Perhaps some will find it useful. BTW, I'm running everything on Unraid (6.10rc8).
I set up collections in Channels DVR. One collection per source of my TV channels. (Youtube, Hulu, Philo, etc). I discovered that you could truly limit the xml output if you use these two things:
I fired up a few different xTeVe dockers, each on different ports (34400, 34401, 34402) because it is extremely finicky with how it handles this input. It would constantly try to duplicate things and turn it into a mess if I tried to use just one docker and put a few m3u and xmltv lists in it. So, I used the above URL's as the m3u and xmltv lists for each TVE source, mapped the channels and used different channel number groups for each. So, 1xxx numbered channels in xTeVe docker 1 are for Youtube, 2xxx for Hulu, etc.
Then, Plex will autodetect each xTeVe instance as a tuner and you can use the appropriate xTeVe output URL's for that. It will auto detect all the channels, get the correct numbers and organize them the way that I wanted. They are sorted by channel number (ascending, my preference) and now I can tell my users that 1xxx channels are for YouTube, etc.
I wish that there was a simpler way to do this, but everything I tried ended with massive frustration for what I was trying to achieve. Is there any chance that we could get Channels to completely separate m3u and xmltv outputs based on source, rather than adding in unnecessary data into them? Kind of like what is done with the favorites filter? I tried the collection=xxx on a hunch and got it to work, but it smartly assumes with those collections that I want to remove duplicate channels, when I actually don't. That way, I can try out IPTV services, see what I like (more stable or higher quality version of a particular channel) and be able to add/remove them more easily.
Hope this helps someone that has similar goals.