I'm teaching a university math course next semester and I've been messing around with a solution to this that my old analysis professor used to employ. Every week, they take home a handful of theory questions. These aren't turned in for credit. But, every week, there'll be a short quiz where one of those questions are asked verbatim, along with a small follow up.

That felt like a very natural solution to me. Of course, worst case scenario, they just blindly generate and memorize the proofs without thinking. But I think the follow ups will catch them in that case. Moreover, I choose to believe that >90% of my students are willing to at least put in the work of scrutinizing those proofs which is honestly not bad exercise in its own right.

I would love to do oral exams directly, the logistics with the number of students I have just make that hard. I know we're trialing it with the less populated special topics courses.