Thank you @PolynomialDivision to fix the GL-S1300 support, I managed to finally resurrect this older device.
Last supported version was 22.03.7.
Sysupgrade didn’t work properly at first. Debricking using the u-boot web interface was also unreliable (seemed to lose link after one minute). I ended up reflashing with the https://github.com/gl-inet/openwrt/blob/openwrt-19.07.8/s1300-factory-to-openwrt.img (2021-07-05 version), reflashing 22.03.7 from u-boot, booting into it, running firstboot, reboot, & sysupgrade to the snapshot.
A lot of hassle, but it works in the end.
The snapshot image doesn’t seem to include luci, so I had to add this manually.