I know there are some retired military members here who have Tricare Prime. At age 65 you must switch to Medicare unless you have employer provided coverage. In any case, Prime becomes Tricare for Life and is your Medicare supplement at no charge. In my case, the co-pay for my meds is less than the cost of a Part D plan, especially using the Express Scripts mail pharmacy. Here's an example of my out of pocket medical costs since October 2022.

October - PET scan that showed mass on right lung...............$0
November - Lung biopsy. Unsuccessful due to lung collapsing. In hospital 3 weeks.........$0
December - 2 days after discharge, readmitted with pneumonia. In hospital 1 week.....$0
February - Different procedure for lung biopsy at UAB. Veery early stage 1 lung cancer confirmed....$0
April - 4 radiation treatments....$0
May 21 - Heart attack, ER, ambulance transport and stent put in. Discharged from hospital May 23....$0
May 25 - Heart attack, ER, and ambulance transport. Unable to stent. Admitted to hospital. Bypass surgery May 30. Discharged June 5....$0.

I have had multiple follow ups for the cancer and surgeries, plus monthly visits with primary care doc. All $0 co-pays.

I realize that there is no one plan fits all out there. I'm only saying that retired military folks should do their due diligence and make sure they aren't spending money on supplements and Part D plans they don't need.