my preferred Android apps, esoteric / highly specialized

display the current charging current/power — Ampere

(This is very useful for identifying low-performing combinations of charger + USB cable. If you want to focus in on just one of that combination, it's possible to get hardware that might help with that.)

general-purpose network diagnosis — PingTools Network Utilities (initial basic diagnosis, traceroute, speed test with latency+jitter+up/down throughput, DNS lookup, ...), Pingmon (latency, jitter)
wifi under-the-covers — Network Analyzer Pro, WiFi Analyzer