The conundrum with 'except mix'

the conundrum with 'except mix'

The passage below comes from a website as follows:

http://www.homegrown.org/group/homegrown-book-club-cooked/forum/topics/week-4-discussion-part-3-air?xg_source=activity

"The sense of accomplishment surprised me. I hadn't done much, after all, except mix together some flour, water, and a little sourdough starter, and then babied it for several hours. And yet—here was this substantial thing that hadn't existed before, this fragrant risen form. I might as well have pulled a rabbit out of a hat ..."

In this passage, I think 'mix' is somehow ambiguous.
Since 'except' is preposition, a word following 'except' is supposed to be a noun phrase. But 'mix' doesn't seem a noun phrase to me. Then, there can be only one possible way to explain this conundrum; 'mix' is infinitive.
But considering 'mix' as infinitive is a bit ambiguous; MIX can be verb of present tense.
Because of the reasons suggested above, to me it seems to be a better choice that we replace 'mix' with 'mixing'.

Am I right in this train of thought?

Regards.