On a positive note no spontaneous reboots you reported initially?
about the 5Ghz it may bear down to the candela driver not being utilized in the TO
there are issue reported with a high number of clients
http://lists.infradead.org/pipermail/openwrt-devel/2019-July/017937.html
though the r7800 features the same Wlan chipset but uses the candela driver instead if I am not mistaken? and suffers it in its own right
http://lists.infradead.org/pipermail/openwrt-devel/2019-June/017781.html