Thesis:

Saint Paul’s Greek is so poor—so densely knotted and infamously thorny—because it was not, in fact, his first language. He was raised in a household that spoke fluent Aramaic, and he also learned very young to read biblical Hebrew.

Thus, while he was technically fluent in Greek, it was actually his second or third language, the way the child of immigrants grows up with a mother tongue in the house and a city tongue outside of it. That would make Paul a savvy code-switcher based on regional, cultural, social, economic, and religious context. It would also explain his letters, as well as his self-identification as a Hebrew, his regular disagreement with known LXX traditions and sometimes evident allusions to the MT, and Acts' explicit depiction of him as fluent in Hebrew (=Aramaic).

Besides standard scholarly assumptions about Paul, I’d be interested to know scholarly reasons to doubt this thesis as at least plausible, if not probable.