The fact it is F or G is irrelevant to what Medicare covers. The coverage is the same for Medicare itself regardless of which supplement you buy. The difference is what part of the costs your supplement would pay that you would otherwise be on the hook for. In other words, if Medicare doesn't cover a test then your supplement won't cover it.
Medicare has limits on screening tests. So, yes, a screening cholesterol test is every 5 years.
However, some things can be done more often for diagnostic or other monitoring or medical reasons. For example, Medicare will cover a screening mammogram for women every 12 months. But you can have a diagnostic one when medically necessary.
Look at this link to find out how often screening tests are done:
https://www.medicare.gov/coverage/pr...ening-services
If it was me and I wanted screening blood tests more frequently then I would go to one of the places where you can order them directly online and pay out of pocket for it and then show the results to my doctor.