First your part D plan. This can be renewed or replaced every year depending upon A) your normal prescriptions you take for maintenance and B) which pharmacy you use or will give you the best deals. My wife and I change our part D plan as time goes by.
Your Part B gap plan most likely will renew you at age 65 and for some the rates may increase or in some cases decrease. You certainly can shop for other plans prior to the month you turn 65 and they may or may not ask you about pre existing conditions. I would recommend at least trying for a better rate plan.
Now, consider this: a high deductible plan may be your best choice. If you look upon insurance as something to cover you in a disaster or major out of the normal health issue, then most people are willing to buck up say $1500 to cover this unusual circumstance. Consider that the $1500 represents the 20% that Medicare may not (may being the operative word) cover. That’s $7500 worth of covered Medicare in one year if all of that requires the 20% to be payed. Anything above that then is covered by the gap policy. For people like me, my gap policy hasn’t paid a dime in the last three years and I have the standard deductible policy. It’s something to consider and we should all know exactly what our gap plan has actually paid.
As for expensive medicines, some are covered by Medicare but they may not be the same medicine you are taking. MS medicines for example, Gylenia is not covered which is a daily oral but (name escapes me maybe Ocravis) taken by injection two times per year is covered. Both have relatively the same efficacy. The Medicare folks can answer which drugs are covered.
Hope this information helps a little.