Well, this may provide some insight. (No, don't click that link, really, you've been warned!)
The current link for AP+STA mode is on the current wiki, https://openwrt.org/docs/guide-user/network/wifi/ap_sta
(Yes, full links are fine to post, especially when they indicate what is on the other end)
https://openwrt.org/docs/guide-user/network/wifi/ap_sta#step_3revert_to_ap_only_mode_on_boot_when_hotspot_is_not_available looks like it addresses the "AP up when STA not connected" problem (however, rereading it looks like only at boot time).
The travelmate package handles this as well, and it might be worthwhile to look into its source to see how it is handled there.