@DHill6
I would not say there are any quirks with the A model. I would say overall it is designed way better than the D. I owned a D for a short time but found that after some use it leaked because the nubs on the base would wear and compromise the seal. Almost disastrous really.
The A has an inner plastic chamber that seals better. The plastic cap though I found overtime starts to wear as well from opening and closing, especially if you over tighten. I had my friend 3D print me new caps with a built in O-ring seal. A design upgrade

I can get you the files if that’s something you’re interested in or maybe he can print you up some too and send them.
I run mine at about the 8-10% h2o2. I mix my own h2o2 from a 35% solution. If you’re running the D at 6% you’re fine to keep it at that. The A is only a larger model. It won’t use more or output more solution than the D does. Your tank will determine that. Factors such as temperature and organics can effect how much solution is used amongst other things.
I totally concur that running one can be helpful. Water clarity is the biggest thing I notice! Definitely no harm to running one properly. I’ve been running it for about 1.5yrs in this tank. If your solution percentage is too high some tank inhabitants will likely be irritated. One thing I have noticed and is a good indicator of an oxydator issue is zoanthids. They are the first thing to be affected and close up. They are my canary and can alert me to a possible leak.
When it comes to dosing I’m running a kalkwasser drip. Daily phytoplankton and fuel by aquavitro. Daily PNS bacteria. I run a skimmer and a reactor with some carbon and phosguard. I also have a large chaeto refugium.
Feel free to ask any other questions. I’ll do my best to help you. I’m not a part of FB either. I’m here on occasion but mainly on Instagram. You can find me @doubleadeejay I find it to be okay for socials. Some good people with good info around there. Plenty of crap too but less than FB haha