https://www.mobapad.com/products/mobapad-m6-hd
And, by god, for the first time I think we've found an amenable solution. I'll do my best to eschew my native prolixity to give you a simple rundown, with some added details in spoiler tags.
Overview
The main thing is: you can comfortably play TATE STGs with your flipgrip with these things. That's the major revelation here, and for that alone it's a recommend from me.
The D-Pad on this thing is actually solid. They use a pretty soft feeling set of microswitches. Tap-dodging feels great, as does steering. I might say that the microswitches might almost be a touch soft for my taste. I think I'd still give my preference to the membrane D-Pad of Hori's Split Pad, but that might only be because I've grown very accustomed to it in the past three years. Regardless, compared to any other wireless option, it's an enormous, enormous improvement.
Spoiler
If you've also been trying to find a solution to the FlipGrip Joycon problem for a long time, you'll know that one of the leading solutions was to replace the shell of Nintendo's stock joycons with an aftermarket shell with a dpad. The solution sucked because the stock arrow-key switches on the Joycons were very stiff, and not conducive to subtle button inputs. For this reason, I was hesitant about the Mobapad microswitches, but now I can confirm that they're much softer. The difference between unplayable and playable.
Spoiler
There's also swappable faceplates for the joycons included that let you install octogates for your analog sticks. I don't have a use for this, but it's a neat inclusion. Faceplates are held on my magnets. Assume it's a pretty easy swap, though I haven't done it myself. The analog sticks on these things are meant to prevent drift, but I almost never play games that require analog stick use.
They're a touch bulky for my taste. Think something along the lines of the OG Split Pad.
When I play horizontal shooters, I prefer the SplitPad on the lefthand side of my Switch and a stock Nintendo joycon attached to the righthand side. Ergonomics or whatever. I tried doing something similar with the Mobipads (Mobypad on left, Joycon on right), but it seemed to cause compatibility issues. Not sure why, but worth noting.
Unknowns
I've had these things for only a few days, so lots I still don't know. Will they be durable? How will the microswitches wear? How much latency is introduced when they're being used wirelessly? I have no clue. Hoping someone more technically inclined might be curious enough to pick these up and measure the latter. They feel fine though.
It would have been really fucking nice to have these in 2020. I hardly play on my Switch at all anymore, but I'm am glad that a solution finally made its way to market. If anyone else picks these up, post impressions.
