The problem you are trying to solve (adding mpeg-dash streams to tvheadend) is a problem that needs to be solved on the tvheadend side (by the tvheadend developers). 4.3 has not solved mpeg-dash yet. I’ve tried all sorts of ffmpeg pipes, and streamlink, and tvheadend just can’t deal with the results from those types of streams. So tvheadend wont work for you at this point. (I’m not sure if any of the other DVRs like Myth or VDR handle it better)
IPTV Merge IPTV Merge combines multiple m3u and epgs for IPTV Simple client, and comes with a proxy to make all the streams go though inputstream.adaptive. (they’re kodi plugin links, so tvheadend could not call them). I did notice I can now pause and resume streams with it (that’s something new). It’s not a DVR, but it may at least help you get your channels into the EPG. You can mix tvheadend for your OTA channels (and any non mpeg-dash streams) side by side with iptv-simple (pulling from the list iptv-merge generates), and they both can sit comfortably together in the same guide.