Old leaves turn yellow. But maybe a fertilization issue? Nitrogen? Cold stress? To the extent the issue is insects, gotta just look.

Did you see the mites with your jewelers loupe? I use a cheap 30X jewelers loupe from amazon to look for the little critters and works very well, get one and you won't need to guess.
I don't believe it's mites and a nutrient deficiency. The condition is called interveinal chlorosis, and when the tissue yellows but the veins remain green. If those are the oldest leaves it's a magnesium deficiency, or from to much potassium in relationship to magnesium levels.
If the icky leaves were middle leaves the deficiency would be Zinc followed by stunted growth in the early stage with the older and younger leaves turning yellow as the deficiency progressed. But it does look like the older leaves have the problem.

For zucchini, yellow spots can often be caused by a deficiency in nutrients such as magnesium or iron. However, if the yellowing is accompanied by distinct patterns or spots, it could also be a sign of fungal diseases like downy mildew or powdery mildew, especially in humid conditions.

It is not downy or powdery mildew I always get that at some point and know what it looks like. I need a 30x magnifier:)

It is the older leaves. I doubt it is nutrients, plants are a lot bigger now and I have not added anything and new leaves are fine. My magnifying glass looks like tiny white spots I think tiny bugs (aphids/mites).