Totally get you, just can't stack these things, ala DP-to-HDMI converter, then another "active" device ala even these edid devices. Sadly what I need is an active edid magic box that takes DP1.2 in, spits out HDMI2.0, supports full 4:4:4 4k/60, and allows me to futz with the edid hard. I spent some time looking around amazon, nada there, but need to expand searching some. These were far more common prior, but not so much now, and def not at 4k res.
I've not had an issue using 6 or 10 foot cables, or a combination of both, with or without dongle - they either work, or don't. The cable solution I'm using now is
an all-in-1 cable, DP on one end, built in converter, and HDMI out on the other end, and 10ft for all (figured I'd try consistency). I still seem to always get one tv, the same one, that sometimes refuses to start, like this morning.
I typically first thing remote both the tv's on, dock disconnected from laptop TB3 port. TV's boot up, no signal. I plug in my TB dock, typically a few seconds later the desktop adjusts and brings up both TV's. Just randomly, and it's not been in a good week or more, the first tv on the first dock port will not start, but the os thinks it's there. I've verified this before with xrandr. I then grumble, go to the dock and physically disconnect the DP port, plug it back in, and suddenly it'll work again, sometimes days, sometimes weeks, until it screws up again.
My desktop would do this with a dongle converter + hdmi cable, some 6ft, one 10. and the displays not starting would randomly change, requiring the same hard unplug/plug at the DP port. I was hoping the combo cable would help this trying with my laptop, but mostly the same.
It's like whatever chinese adapter chip they use are clones of a clone of a clone at this point, and just somewhat flaky crap. Like what you get with flaky serial adapters, sd-radios, and other cheap electronics from wish or alibaba, but the bad clones are so pervasive now, you can't NOT get a piece of crap part anywhere.
-mb