I find that these insoles from Shoewawa fit and support my feet perfectly. Much betetr I find than superfeet or powerstep ones.

Heres the link

I'm in the very flat footed camp too. Walking in sand, my foot print looks like a shoe with toes. No arch here. I have been really happy with the New Balance Vazee Pace.

Here's something worth thinking about though :

It is very possible that your Achilles pain after 3 months is actually your body signaling to you to adapt more slowly or take it easier. Most well fitting running shoes should get more comfortable over time, not less.

I'm going the opposite direction of you after 25+ years of running in really built up shoes, and now going to barefoot vs no-cushion shoes (mostly <$20 water shoes, actually). The transition is hard - lots of new aches and pains and warnings from the achilles. It however, would be incorrect for me to instantly conclude - 'pain, gotta be the shoes/style, time to quit.' I actually WANT to feel all those aches and pains, and even skin blistering from the barefoot style - it forces me to be MUCH more aware of overtraining, overuse injuries, and other typical injury-related things that plague runners who just seek out more and more protective shoes that often mask the underlying problems that they're not addressing. I know this well, as I've overtrained the run quite a few times in the past, and I'm trying to now build a more solid foundation with less reliance on plastic external pieces and more reliance on the natural human gait which is supremely adapted to running. It's been really eye opening to see that some of the barefoot aches and pains I have now gotten acclimated through (and no longer feel) were clear signs of my body telling me to slow it down or ease it up, whereas with the big protective shoes, I would have blasted right through most of that. Also lots of improve on run technique, in particular downhill landings (I'm now using the WHOLE leg and foot as the spring, not just stomping on the heels), but that's a different story.

Flat as a pancake and have been running in Altras since about the time the first model came out. Wouldn't consider anything else at this point.

I'm very flat footed (fallen arches) and best I've found so far are OnCloud Flyers. I've tried NB, Asics, Newton and the On's really do seem to make the long runs feel a lot less tiring.

^^^this.

I have super flat feet, but my gait is very natural. I've had success in neutral shoes, and supportive shoes tend to cause me more pain than anything.

I didn't used to know this though, and I had to try dozens of shoes before I figured out what I liked and didn't like in a shoe (not the most streamlined way, I know). My general rule now is try them on and decide if they feel like they fit your foot. If they don't feel good standing, they'll likely not feel good running. If they do, try running a few runs and see if it allows you to run how you'd like.

FWIW I've ran in Newton Distance S shoes for 6+ years and enjoyed the combo of minimal cushion and mild support. However, I have since switched to New Balance Zantes, New Balance 1500's for track work, and Asics Noosafasts for races.