Just to add my tuppence worth, with a ray of hope for those of you who haven’t yet cracked their version of the issue.
My 2017 iPad Pro (256 Gb) suffered what I now suspect was the same problem. It discharged rapidly down to 2% over the day after the update, and then no matter which cable (Apple or 3rd party) or which charger (Apple or 3rd party) I used, it wouldn’t charge. Tried hard resets (but not factory resets).
I found that if I left the iPad on charge for *a whole week* (having turned it back off after plugging in), the battery crawled back up to 30%. I figured that 30% was enough to try a full iTunes backup on my 2010 (haha) Mac mini the old-fashioned way. I hooked up to the mini and unexpectedly I got the lovely green battery *and* the charging bolt symbol. That was weird. Tried the cables and chargers again — no charging. Plugged it back into the Mac — charging.
So, if you can’t wirelessly charge, or use your cable to a Mac, do try a seriously long charging session off the mains anyway in a device-off state to get enough juice back to try an iTunes sync/charge. Or — as it turns out — possibly a regular charge. After an extended session on the mini to get the iPad back up to 100%, I found that just one of my 3rd party cables on a 3rd party charger did work (while the others still didn’t).
I’ve since been running the iPad all day off that cable + charger combo with no problems for a good couple of weeks. Having read the posts here, I thought that I’d try one of my other (non-working) cables in the working charger, and it also seems to be working again now. The same cable also works on a genuine 2.4A Apple charger (which hadn’t worked before). But still not on my other 3rd party charger (which does, however, charge lower-power devices like my original iPhone SE).
In combination, that does rather support the idea that a) the CPU was exhausting the battery soon after the update, and that over time, it sorted itself out again; and b) perhaps the recharge draw during that time is too much for the 2.4A circuit in cheap 3rd party chargers, which promptly burn out (leaving the 1A circuits intact).
So there may still be some hope for you all to rescue your lovely tech.
On the downside, I did find all this out after I’d already bought a 2020 iPad Pro to replace my “dead” model. On the upside, I now have 2 functioning iPad Pro’s, which means I look like a 2020’s Neo when I’m zooming on my iPhone + working remotely on 2 IT systems.