I tested the 23.05-SNAPSHOT version (specifically, OpenWrt 23.05-SNAPSHOT r24026-6fadcee50b / LuCI openwrt-23.05 branch git-24.212.79335-cdbe903) by using web-based attended sysupgrade and hacking the select field value.
The "6.0 Mbit/s" display bug (possibly a consequence of some power-saving mode not enabled previously) is still there.
The "impossible 960 Mbps rate on a WiFi5 device" bug can no longer be reproduced.
I also have a speed problem on a wired client connected to the repeater:
For comparison, here is what I get when the repeater runs 23.05.3:
Note that no 2.4 GHz transmission is involved at all in any of the above tests. The main router (Linksys E8450) runs OpenWrt 23.05.4.