Forgive my cluelessness but I don't know what "side addresses caps" / "sdc side-address" means - ?
sdc = small diameter condensor microphone - "pencil condensor", not a large diameter often switchable pattern condensor microphone, typically phantom powered)
"Side address" means the diaphragm faces out from the side of its tubular housing, rather than facing out the end of the tubular housing, which is refered to as "End address"
One nagging question is will I have an easier time overall (fewer custom cables required, more compatibility with everything else) if I upgrade to a recorder with XLR inputs and phantom power?
IMO, in general the less components and less cable connections, the more reliable and more stealth-able a rig will be. There is a lot to be said in practical terms for eliminating extra stuff between the micrphones and recording device where possible. Countering that is some minimal amount of "stuff in between" is often required to run the gear one wants, or achieve the highest levels of quality.
Capsules using the new small modular amplifiers directly into a recorder that can provide phantom powering is a way of achieving that simplicity, eliminating the stuff in between. Recorders that provide phantom are somewhat larger though, so there is some trade off in size and other factors in the stealth equation.
So you'd get a used / refurb phone, not set up any cell service on it, but use it in airplane mode with wifi to get it set up, put the DPA app on there, and transfer files - ?
Is it really not possible to use one's primary phone to record?
I'm not a phone taper, but I'd feel more confident using a dedicated, perhaps older phone, or non-phone ios device for recording, if I were to go that route. Can use the primary phone, but there is plenty of potential for something to go wrong and screw up a recording. I tend to gravitate to gear that is simple and works without complication, perhaps because I run what most here consider to be ridiculously complicated multi-microphone setups, so keeping the recording chain, interconnects, and devices simple and dedicated to the purpose a hand makes for a valuable overall strategy for me.