Sadly, we have not tested the latest OpenWRT versions on our device, meaning we cannot guarantee it will work without any issues; however, if the firmware could not be flashed through the WebUI, it can be flashed through a console (SSH) as it was addressed here with "sysupgrade -F /tmp/FILE_PATH" command.
Regarding the additional questions, unfortunately, I would not be able to provide the number of possible hotspot users with such functionality as we have no test reports regarding this.
As discussed, our device has a hotspot-captive portal which can be found Services -> Hotspot -> General. More information about its configurations can be found here.
As an alternative, you could use Coova-chilli for the captive portal, since it is not a native Teltonika Networks package we would not be able to guarantee that it will work smoothly, but nevertheless, you can give it a try.