I am toying with idea of changing the whole concept of SFT composition. Instead of weapons there can be units (about battalion size). So a SFT called "Infantry" should be considered as a unit which has small arms, MGs, support weapons (mortars and infantry gun) component already in it. Infantry (mot) has also truck component in it; Infantry (Arm) has halftrack component. Light Recon Btl represents a mix of horse recon, bicycles, motorcycles. Armored recon Btl includes armored cars and/or halftracks. Artillery Btl can be either horse drawn or motorized etc. If unit is motorized it moves better but costs more raw an consumes more oil.
So instead of producing infantry+weapons+transport one will produce functional units.
The infantry research tree should include the possibility of upgrade small arms, MGs and AT weapons (one can choose in which order). So you may have Level I infantry with upgraded AT weapons or MGs or both etc. If all three have been upgraded then you have Level II Infantry and one can then again upgrade the all three components.
For example if one researches Infantry MG upgrade, then all the infantry units which have (theoreticaly) MG component can be produced with the upgraded MGs.
I have thinking also similar changes in armor and air research tree. Here's a visual representation of the armor research: