- OST can make bunkers reinforced/health
- SOV have FHQ.
- USF have ambulance.
- OBW have trucks with reinforced/health.
It is a design problem of the 4 factions.
Wrong. You're comparing non-doctrinal things with doctrinal FHQ.
Soviet have M5 to reinforce. Which costs less fuel, mobile and can be upgraded to M17 for AI/AA.
Still it costs fuel and forces you to tech shitty tier (unless you play Lend-Lease Tactics which also gives a great medium tank so no T3 needed).
Closest thing to M5 is Wehrmacht SdKfz 251, which cost fuel and can be upgraded for better AI too. BUT, as you said, Wehrmacht have bunkers which cost no fuel.
In 2v2+ USF player can decrew their Ambulance truck for soviet to take it. USF get medics and soviets get forward reinforcement and healing.
I see no faction design problems here. Problem is in Urban Defence Doctrine and in knowing(not knowing) how to deal with it.