What to do if a journal article is too hard to review
Decision based on maths. If an overhaul of the mathematics would significantly reduce the time required for an expert to understand them, then you should state that in your review and that might well be a reason to require major revisions. On the other hand, if such an overhaul would only help non-experts, then that might well merit a mention in your review, but I don't think such an overhaul is necessary for acceptance.

Reviewing. Two weeks is ~4% of the year. Committing that amount of time to a single review (as would be required for you to study the details of the mathematics) seems unreasonable to me. Perhaps you can raise this with the editor. They might well suggest that you disregard the details and focus on other aspects of the manuscript, they can always ask another reviewer to check the details.

Scope. To determine whether the manuscript is in scope, you could compare the volume of material on control with articles published by the journal. If you think it is out of scope, then perhaps raise this with the editor.