ぐにゃりとする VS ぐにゃりとした

僕は一刻も早くこの場から去りたくて、でもそのためには早く片付けなければいけなくて、濡れてぐにゃりとするフライドチキンやおにぎりの食べ残しも素手で必死に掴む。

Hi. For the red part, is there any difference between とした and とする? I think they are the same in that both mean "feel soft".
Thank you.

In this context there's no appreciable change in nuance depending on which is used, no.

The difference is the simple difference between perfect and imperfect tense. The former is framing the friend chicken as already having the characteristics of being squishy/mushy, while the latter frames it as something that has yet to happen (in this case, not because it's not already in that state, but because the speaker is framing the whole sentence in the present/future as if he's narrating it as it happens).

But again, yes, in this case it doesn't change the meaning of the phrase in a meaningful way.

Thank you for explaining the nuance, bentenmusum-san.