I sort of like some of these ideas

I think two things are evident:
Once the crates are dropped, or the train pulls into the station the buildings STILL come back too fast.
Interdiction of supplies needs to be more 'predicatable' (read as 'preventable')
Back in (cough) that other game they had options for what cargo you were bringing to a field. You could fly in fuel, ammo or cargo (Cargo simply repaired buildings)
And while they did have the concept of auto-resupply of fields, where the fields got resupplied by drones (gooneys in this case) the drones didn't start heading out until the target was damaged.
So we could have the trains only leave a depot when a field was damaged, but I worry this will take far too long. If the trip from a depot only takes 30 minutes, then I'd say this is okay. But anything more...
Also, it's not quite right that fields / HQ come up instantly when supplies are dropped. So here's an alternate:
All fields have 'supplies' of 100% (based on the health of the town AND outlying warehouses (new structure, these are placed around a field / HQ just like towns)
Regrow rate is set to 15 minutes * the supplies on hand. Dropping supplies
a) regrows the town / warehouses instantly
b) allows for EXTRA supplies to be built up (max 400%) and extra warehouses appear.
c) These warehouses CAN be bombed reducing the extra supplies on hand.
d) Extra supplies get used up to repair damage until the 100% base is reached. Supplies never go below 100% when used to build stuff.
e) Extra supplies get used up over time (this means keeping a field over supplied takes constant work)
And you'll note, having 400% supplies only means the field will regrow in 4 minutes.
(and it'll use up some of these supplies in the process )
And it gives the attackers a chance to kill the warehouses and slow down the building process.
But it also gives the defenders a chance to 'shore up' a field if they know it's going to be attacked soon.
Mind you, to balance this out, convoys should carry more than 100% supplies.
When the train pulls in the station, expect the field supplies to be bumped to 200%
This gives the attackers only about 7 minutes to take out the town / supplies before the field grows back and the supplies are back at their default 100%
And if the town / supply warehouse can be completely destoyed (and kept that way) the field will *never* repair.
Just a thought.
~Lemur