If I'd been trying all day I think I'd be a lot frustrated
Again, not an expert, bear that in mind, I do talk rubbish with complete conviction sometimes, but anyway, generally Emby works well but I don't think its really set up yet for thousands of channels. Normally if you get the channels and the guide from the same provider it should be OK and Emby should match each channel to its guide info. If not you have to map each channel to its guide entry manually. Don't know if thats necessary for you or if you've tried that already.
I have an old picture which looks something like this (though as mentioned, I do my far more limited iptv stuff in NextPVR)...
Obviously you don't want to that for all those channels if you can avoid it.
It might be worth trying if possible with a subset of your channels, if there's any way to pick out a few, like say 20, but am not sure how to do that.
Again, just guessing, maybe its busy in the background updating the guide (if you're lucky).