9 times out of 10 it's the battery, or a cable. Even if you measure good voltage with say a multimeter that doesn't mean that much if any actual current is going around. There are some common cables and of course I'd suggest pulling the battery and bringing it in for a check, or get a meter off Amazon designed for the job.
https://www.amazon.com/dp/B07Z67MMGC...d_asin_title_7
Sounds like you're already thinking down that path. I'd say from like 15 years of browsing this site, it's ALWAYS battery (or cable) related when that much stuff goes nuts at once.