One ground per segment or a shared ground back to the controller, what's your take?

I went the other way on my 14ft megatree and just ran one chunky ground up the middle, 14awg from the supply, with power injection at the top as well as the bottom. Sixteen strands hanging off that and no flicker at all, even with big white flashes across every segment. My theory on why your first setup glitched is that the shared ground was probaly too thin for the current, so every segment lit up shifted the ground reference a bit. A properly sized ground bus fixes that without needing a pair per frond.

Where I do agree with separate runs is data. Long data lines next to each other pick up junk, so I twisted each data wire with its own ground and that cleaned things up on a fence build I did last spring. For me the deciding factor is how much current each segment acutally pulls. A frond with 30 LEDs is nothing, but if you get up over a couple hundred per segment or the runs get past about 5 meters, injection points start mattering more than how many wires you ran.

On the status strip, I did exactly that inside my controller box with a short 8 pixel stick showing WLED state, and it's oddly useful at night when I'm wondering why nothing is responding. Cheap enough that I'd just do it.

For what it's worth, on my tiki-torch palm build I found the flicker wasn't purely a ground problem. I had everything on one shared ground too, and what finally fixed it was cleaning up the data side. I put a 330 ohm resistor right at the first pixel of each run and added a sacrificial pixel near the controller, since long data lines into bright white flashes were corrupting the signal before the ground even came into it. Once I did that, the single ground backbone worked fine for me. That said, I did something a bit different for the wiring itself: I pulled an old Cat5e cable up the trunk and used the twisted pairs for data and ground per segment. Eight pairs in one thin jacket beats a fistful of loose wires, and it looks tidy inside the trunk. Each pair can carry a fair bit for a short frond run, and twisting cuts down on the noise the mic wires were picking up. For figuring out whether you need beefier copper, I just measured voltage at the farthest frond while running full white on everything. If the tail end dips noticeably below the supply, that's your cue to fatten the ground or inject power at the far end. And the status strip in the enclosure is not silly. Mine blinks different colors for boot, wifi connect, and errors, and it has saved me from dragging a ladder out more than once.

Did a similar rebuild on a grapevine trellis last year and the wires weren't the problem at all, the connectors were. Every joint I made with those push-in solderless clips worked fine on the bench, then went flaky after a couple months outside. Redid all six junctions with heat shrink butt connectors and it's been two seasons clean. I do low voltage work for a living and I figure roughly one in ten rushed joints comes back to bite me, so I stopped rushing them.

One extra use for that status strip inside the box: map a color per output on a spare preset. When something goes dead mid-season you can tell in two seconds wich port it is instead of unplugging stuff in the dark.

This thread is making me rethink my headboard build. I've only got two segments on that one, but I get a bit of shimmer when both hit full white at the same time, which sounds like the same thing you ran into with your fronds. My runs are short so I never bothered with injection, but maybe that's my fix. Do you add injection at the far end of each segment, or partway along? Also kind of relieved the status light thing isn't just me, might steal that idea for my controller box too.