Tbh a lot of the timing issues have to do with the Luchs and OKW mechanized. The luchs hits the field at a similar timing to the 251, yet is far more durable. This necessitates the AEC(and stuart) bejng available at the timings they are, which correspondingly makes the 251 tough to keep around. Push back the luchs timing and it makes a lot of sense to push back stuart and aec timing.
I also want to point out that the flamer halftrack is far more lethal against infantry then the luchs, but trades durability for it. I dont think the 251 should get much by way of buffs lest a unit that already has devastating potential become overpowered.
My suggestion would to make mecha cost the same T1 move the extra cost to engineers upgrade that also take longer to complete so that Luch can be delayed.
As for FHT my suggestion would be to to make it more durable but move AOE from autofire to an ability adding AOE for a limited or using a "grenade" type mechanism or using the Centaur strafe (as AE has done for WASP in his mode).
Flame vehicles would be allot more balanced if their damage was timed limited.