28nm chips is just about suitable to run frontier models

I doubt it. 28 nm is 4-5 generations back so inferencing would need a large number of chips with very high power consumption. Maybe you're thinking more of 7 nm which is what Chinese fabs have; it seems to be OK for companies like Huawei.

And so if I was a mid-level State would it be worth while to take my nascent chip industry and push it out to build a 28nm foundry and supporting eco-system.

It never reaches breakeven so you'd have to provide billions in subsidies per year forever. The sovereign chip stuff only makes sense for the US and China; even the EU probably isn't large enough to make it work. A single country definitely couldn't.