Good Job, Wasserpest! I see You've realized the short lifespan/value of the plastic syringes. I learned that with reptiles yrs ago, but always thought it was probably the soaking in alcohol that degraded them.
Did You ever get #2 (Test Tube) sorted out or are You still working on it?
I just bought 2 peristaltic pumps--which I think I will really like, but they are cost prohibitive for MTS! I think the Air-pump, water-pump and Aqualifter are probably the best systems--for the money! They offer A Lot
On the Water pump system--I say: skip the "powerhead" and get a small pump like a Via Aqua or Rio. My little Via Aqua 480 will pump about 59" head (straight-up) through 3/8"ID tubing---for only~$10-12. Reducing/controlling the flow isn't too hard.
I didn't even see that post until just now. Basically for dry dosing I think the Lifegaurd Auto-feeder beats anything else hands down. IMO, A lot of the things You listed either just won't work; or are way too expensive, put up too many stumbling blocks or both. Great Effort-Yes! But I don't think people that want to auto-dose really care whether its dry or wet. So, expensive dry solutions are pretty much out when we can have so much control via peristaltic pumps (x2) for under $200. With familiarity the air, water and aqualifter systems can be very reliable and offer Good control for --$25. Its Good to see that You are thinking outside the box--but its a tough challenge!
Either way, I've seen most of these develop during the couple of yrs that I've been around here---so who knows what we will see in a couple more yrs!