Thanks for your help- just to close the loop, I was able to recover my router, and I’m including the steps I had to take in case someone else runs into a similar issue.
For some reason I was never able to get the UART terminal output working (all of my UART adapters are apparently based on the CH340 chip and I have successfully accessed uboot terminals and unbricked other openwrt routers with it, and I have a different one arriving today, but now it’s not needed at least for this issue).
I was finally able to access the uboot web interface on 192.168.1.1, and I was on the older uboot which did not offer the option to enable/disable the router switch for booting into NAND or NOR.
I tried uploading the 4.3.25, 4.3.18, and 3.2.16 firmwares from the uboot page and every time I received an error message that the update failed or could not be applied (sorry, I don’t have the exact verbiage).
Then I decided to try and update the uboot firmware.
I used the directions on this page: Debrick via Uboot - GL.iNet Router Docs 3
and I downloaded the firmware uboot-gl-ar300m-20220216-md5-97ff7bb80cbf129fa21f34ded1559ff8.bin from uboot-for-qca95xx/bin/uboot-gl-ar300m-20220216-md5-97ff7bb80cbf129fa21f34ded1559ff8.bin at master · gl-inet/uboot-for-qca95xx · GitHub
This was successful and now I could see the newer uboot web interface, but unfortunately, none of the 4.3.25, 4.3.18, or 3.2.16 firmware updates (I made sure to be using the uboot .img files) would install and reboot correctly. Each time I tried to install them, the router would go through the motions of installing and I would see a bunch of gibberish on my UART connection, but it kept rebooting into uboot.
I found this thread which described a similar situation, so I decided to follow those suggestions: GL-AR300M16-EXT stuck in uboot
I downloaded the vanilla openwrt firmware https://downloads.openwrt.org/releases/22.03.0/targets/ath79/nand/openwrt-22.03.0-ath79-nand-glinet_gl-ar300m-nor-initramfs-kernel.bin and used firefox and flashed it in the uboot web ui and this time it booted into vanilla openWRT in recovery mode, and I was able to download the sysupgrade firmware openwrt-ar300m-4.3.25-0331-1743423889.tar and flash that from within LUCI, and the router rebooted into the GLiNet UI where I was able to reconfigure it successfully!
That was probably way too much work and too many hours of troubleshooting for a little router that could be replaced for ~$30, but it’s satisfying to save a versatile device like this that can be quickly set up as a wireless bridge or IOT AP/etc.