You keep repeating this and yet you've never really offered anything resembling a concrete proposal on how to actually "make it useful"
-> Remove Panzer Authorisation requirement but make build time 30seconds longer and increase price to 100 fuel
-> Give it to OKW Luftwaffe and Fortification Doctrine and merge that mini flak emplacement with another unit or just remove it altogether. It's thematically wrong but would make the doctrines more interesting and fun to use.
-> Buff the stats a little but keep Panzer Authorisation requirement, like for example shorter build time and/or +80 HP at vet 2