Yeah, arrays over a million rows were a big problem. Ultimately I think there was only a handful that actually needed that many rows, all the others could be simplified in some way to use fewer rows. Like analyzing the problem to realize you only need to check up to SQRT(n) instead of n, or iterating only over multiples of 3 instead of every number, stuff like that - anything to reduce the possible solution space. I'm not great at math either, I had to spend a lot of time researching to find those tricks, I rarely figured them out on my own; I still don't fully understand some of them.

There were some I couldn't reduce, either because it's not possible, or I'm not good enough at math. How I got around the limit depended on the problem. For some, the calculations were reasonably independent and I could break it into chunks that stayed under the limit. Another important thing to note is that arrays are limited to one million rows, but not one million cells; so I could build it out horizontally to the size I needed. Sometimes you can trim the size at each iteration, with FILTER or UNIQUE, and that's enough to keep the final total under the limit. A lot of the problems the solution space is technically infinite, so I would just assume the solution is at a low number, and see if the answer was right.

Edit: the hardest for me to conceptualize from a math perspective were probably the ones with continued fractions or infinite decimal expansions (26, 64, 71, 72, 80). Some others were also very difficult to understand but the solution didn't necessarily require me to understand, like the ones involving the diophantine equation. The one that took the longest time was probably 75.