Did you also update via the TP-Link UI? Do you remember what stock version you updated from?
As an update, I tried TFTP on all the versions of stock firmware I can find using internet archive (from 190403 all the way to newest 211022, total 11 versions). The same problem persists (TFTP succeeds, but update not installed).
From a long search, the Archer C7 page has a section that talks about product_id and product_ver comparison that may be done during TFTP and may be the issue. I will next be trying to install some older versions of Openwrt and if that doesn't work, will try to figure how to modify those fields in the firmware files.
I also tried erasing the jffs2 partition and it shows in the kernel log the same Newly-erased block contained word errors mentioned in OP.
umount /rom/overlay
mtd erase /dev/mtd5
Worried it may be a kernel bug like below, in which case not sure how to fix:
https://forum.openwrt.org/t/19-07-2-mount-root-failed-cp-a-tmp-root-rom-overlay-not-a-tty/61980/3