Current OpenWrt versions, with new kernels and newer wifi drivers and newer packages and ... consume a bit more RAM and flash, so that older devices slowly get unusable.
You device is resource constrained regarding both RAM and flash.
Hmmm, I did install the dawn and updated all the software and then made it a dumb AP and disable the firewall, ipv6, and deleted the wan Interface. that's all and deploy it as a dumb AP after a few days that happens the memory is being used by the hostapd process. in the recent version the CPU is the one who handles the process. but in this latest version, the memory is being used by the process.
I'm using several Archer C7s as access points with Dawn and don't have any memory issues (only 67 MB used of 120). This access point has been up for 70 days.
FWIW, on my C7/A7 group of dumb AP's, without Dawn, typical memory use is usually 50-60ish Mb available, 40ish-50ish used. Also getting long uptimes without issues.
Just noticed this in re-reading, does that mean you did that to everything showing up in the update list? That's not recommended... see other threads on why it's not. Yes, people do it, and a lot of times it's not an issue. Other times...
Perhaps changing to the updated version of hostapd is causing the issue? I'd try again, reflashing with the stock stable release, not updating anything, and see if you still have the issue.