Personally, my take is that surzhyk is not a single variety. It's an umbrella term over multiple different varieties, grouped by origin: either Ukrainian with some Russian influence, or the opposite. With surzhyky from different areas being vastly different from each other.
And, if I had to take a guess, most surzhyky (surzhyks? How am I supposed to pluralise this in English?) of the 1930s were Ukrainian with heavy Russian interference, while the ones nowadays are the opposite. Roughly speaking, the "base" language being the one people are shifting from, and the superstrate being the one they're shifting to.
Some might argue those varieties are creoles; I think the label "creole" gets messy when dealing with varieties that have some mutual intelligibility, like Ukrainian and Russian (or Portuguese and Spanish). Because one of the main features of a creole is "gluing" features from both parent languages with local innovations, but they aren't required if there's mutual intelligibility and a lot of the grammar is shared.