The problem come from the fact that the doctrinal DSHK is better then the Maxim HMG.t
So 1vs1 maps it's rather normal that you want the best HMG, because it will have a greater impact.
If the DSHK is become similar to Maxim or a .50... why does it exist?
Nerf it to the level of the non doctrinal MG42 ? why not ?
The problem i see in the solution that all call in should be tight to a tier building is that in certain case they are a solution for fuel scarce situations. (where you dont want or can't build tier 4 as an exemple.)
the Lend-lease doctrine is made for those fuel scarce situations.
So maybe it's best to increase the mp cost of the call-in as someone said.
And it other case it's best to be tight to a tier.
The design of commander with call-in units must be thoughtfully analysed to find its original purpose.

I don't care what its original purpose was, if it isn't working and is broken, it should be reworked. Or do we need to thoughtfully reanalyze the Tiger Ace and get it back to its orginal purpose?
Calls in are ruining the game since launch, that is why tigers, Is2s and other heavy tank call ins were limited to one at a time and the double T34-85 call ins tied to teching.
All call ins need to be tied to teching. And yes, even the ones from the factions you defend with every post.