[Marcello]

"As a self-propelled artillery vehicle it also seems like it would've been a cheap alternative to more advanced SP 155mm guns."

If you want a cheap SPA I suppose that those truck mounted artillery pieces they make nowadays would be a much better option.

"he hull seems to be lower profile than the standard M113, and as a tank destroyer or tracked assault gun it really seems no worse perhaps than the ASU-57, ASU-85, M56 Scorpion or M50 Ontos."

Those vehicles (and I would include the Jagdpanzer Kanone in the list, which seems the closest thing to it for me) are obsolete for the most part so being no worse does not cut it. I suppose that if you were desperately short on tanks such vehicle would be able to provide some cheap and armored firepower support to infantry units, as it still beats throwing men against machine gun nests. Since MBTs are plentiful today there is no reason to build something like that. Airborne forces are better off with some Sprut lookalike.
I think that there may be a niche for assault guns in urban combat, but that requires a different sort of vehicle.