The annoying thing though about that doctrine, was in practice the TD's were lightly armored, and more redundant than anything. Sure, it's great to have a lot of self-propelled AT guns without much weight, how about tow those in light halftracks and have those also carry their crews, provisions, & ammo. ...
Tactically the towed AT guns were less sucessful than the SP. They lacked the ability to move rapidly forward as a support weapon & in the defense could not quickly displace if the situation required switching positions. The better part of my career was in the artillery & the SP weapons were usually preferable from a tactical standpoint to towed.