My experience with amanos is similar. I've made several improvements since the last batch died: added a spray bar, have a lot of flow for gas dispersal throughout the tank, 2/3 of the surface has small ripples while the other 1/3 has individual boils from the spray bar jets.
Recently I added a few amanos and ghost shrimp, drip acclimated, etc. The ghosts are active; the amanos stay hidden, assuming they're still alive.
PO4 10+ (burning off to 1-2 ppm)
I keep a koralia 1050 running most of the time which provides a good amkunt if agitation throughout the tank. Plus the wet/dry sump, O2 levels should be high.
I keep them just fine (they even berry up) in soft water at around 74F-78F in my 20L with nothing more than an Eheim 2217 with an in-line atomizer for CO2 and a Vuppa for surface skimming. However, my tank is extremely dense with plants. I'd like to think that oxygenation plays the largest part in keeping amanos alive in a CO2 injected setup. Lots of plant density + no surface film = lots of oxygen. If your amanos have a bluish hue to their innards when you find them dead...too little oxygenation. However, that is a really low TDS so you may want to up the gh a tad bit.
I think your Vuppa plays a bug role in high oxygen levels. I have a modified filter guard in front of my sump overflow which I think may have increased the surface film. I never find the dead bodies with the amount of hardscape I have. I remineralize my water everytime with a water change (maybe once every 2-3 months as I'm dosing PPS-Pro). I should have a GH of around 5-6.