Hmmm... yea I definitely think Inert/Contact is the most relatable/accurate of the subtype systems, but with a few modifications...

I think that Inert/Contact subtype should alter two traits:
(Slight)Strengthening/Weakening of Inert/Contact functions (aka the functions that are strong in the type in the next quadra sharing the same function you are enhancing; INFj-Fi strengthens SF because ISFj)
(Slight) Increase/Decrease in Value of functions based off nearest Quadra (INFj-Fi would value Se, Ni, Te, Fi more and consequently devalue Si, Ne, Ti, and Fe.)

Why?

Because I like the idea of a type spectrum, it makes much more sense than having every type going off in its own direction when affected by a subtype, when there's literally two very similar types right there to transition to.

But at the same time.... I could see how one might be stronger with their leading or creative function and value the functions with the same orientation more, rather than adapting to the quadra of the subtype. ... like if I were Ni subtype I wouldn't think I'd be more receptive to Se but maybe be more in touch with Si? idk.

ANYWAYS...

I wouldn't know what my subtypes are, I don't even have a Pre-subtype type yet. ;_;