Help diagnosing system crash after flash stock+factory reset+wipe?

Long question, sorry (...f****** long repair process)

I've been trying to fix my wife's phone for weeks now. The short version is that I can now flash a stock G930V firmware using Odin, phone boots, proceed through setup etc. but some time later (a few hours to a couple days at most) the phone will crash to a black screen, won't boot, doesn't seem to want to charge (meter currently shows it drawing 0.2A at 5.2V), and I can't get into DL mode. Sometimes after leaving it a while, I can get it to charge and/or enter DL mode after connecting to my PC. Then I'll flash a stock firmware (including the CSC not HOME_CSC file) boot into recovery, wipe data factory reset & clear cache. Phone boots, go through setup, (most recently we tried not importing apps or settings -only thing she installed was Tumblr app after ~12 hrs) it stays usable for hours to days, and then crashes again.

I'm sorta lost on how best to proceed, or what could be the root cause, so any and all advice is appreciated. I'm not sure I can even access the phone through my PC when it's crashed out, but if so are there any adb commands or debug apps that might shed some light?

How we got here:

Originally she had system crash after an OTA update (she thinks). We tried a wipe + clear cache, phone wouldn't boot, or got stuck in a boot loop. I was having trouble getting into recovery or DL mode, so I opened the phone to pull the battery (screen was cracked beforehand and died upon opening). Replaced that, meanwhile noticing the battery voltage was low (~2.4V) but it took a charge so I left it alone. Also noticed damage to power flex board side connector, but I checked for shorts and continuity and it was fine so I left it alone. New screen installed I flashed a stock firmware (latest 7.0 iirc) through Odin, phone boots but we can't get past setup wizard because of frp lock (either forgot the username/pass or too many password changes in the weeks the phone was out of use, even after verifying the credentials it was locked out from excess attempts).

So I proceeded to try every bypass trick I could find, changing bootloaders and upgrading and downgrading between 6.X and 7.0, so many times I couldn't even tell you, until eventually I was able to get in to enable USB debugging and used adb to update android account info. Get through setup, phone works a while, (maybe did an OTA update) then some time later crash..

Now Odin fails at "get PIT for mapping" so I try flashing several stock firmwares (with re-partition checked) until one works and the phone boots and so on and so on ad nauseam. A few days ago I replaced the battery (old one was DEAD dead) and that got it out of it's blackout state, and yet here it is on my bench dead and lifeless once again.

I'm gonna check the power flex connection again (it was fine but that was a few mating cycles ago), but if that doesn't do it I'm totally at a loss here. Do androids get malicious rootkits? Wife got one on her laptop around the same time this all started but I figured that for a coincidence (laptop was an imposter tech support remote desktop ransom scam).