I retired about 20 years ago and thought the TriCare Supplements were the way to go - - then I did the math. I switched to TriCare Prime and will stick with it till I have to go with TriCare for Life.
I believe the major factors in determining which is best for you is the old adage, location / location / location...Depending on where you live, your proximity to a Military base (with Clinic or a Full-fledged Hospital) and if you have “special medial needs” all come in to play.
In other words, there is no one best answer - - it depends.
Fun with math....
TriCare has a $3,000 catastrophic max (catmax)
If you buy a supplement and it cost you $250 per month - you will pay $3,000 a year in premiums - thus it can only cost you more, never less than the $3,000 TriCare would have charged you if you reached your max.
Prime is not much different price wise... I pay an annual fee, but I find it lets me in at the best doctors in the area (Johns Hopkins, UMMS, etc...)with little out of pocket expenses.
I see no value in ever buying a TriCare Supplement. The annual premium equals the CatMax - - so they 9the insurance company) pockets the difference, and you lose. (Suckers bet) IMHO