Not a direct answer to your question, however, If I can recommend, if I could only choose a couple , the data points if exploring the various prep programs I'd pursue are:
To the prep school contact: "What percentage of people who attend this prep school are accepted into/ attend an academy after completing this program? Please anonymize but show me actual numbers from the past 5 years".
I'd also use the transfer portal at say the top 3-5 civilian colleges your young adult may attend if not offered admission to an SA after a prep program year, to confirm. To the civilian college you may transfer to as plans B, C,D: "Exactly what credits for what specific courses will I be granted if I transfer to your university after a year in prep elsewhere".
There's a lot of assurance across the various prep program sites about the individualized championing / connections / prior success stories, customs and courtesies foundations, readiness you will achieve - yes, yes, great, and yet... "What percentage of people who attend this prep school are accepted into/ attend an academy after completing this program?" One of my son's good friends attended a prep program after graduating HS in 2019. He did not gain admission to the SA he was pursuing and wasn't the only member of his cohort to have this outcome. I'm sure there will be some who ad hominem assume he dropped a ball, but he didn't. Nor was he an outstanding standout in that program - ,middle of the pack... the civilian college he attended took little credits from his prep year - the same result he found when explored options at a few colleges. He'll graduate college next month after 4 years a regular college, and 1 year of prep. He will make good money in the maritime industry but I wish he had asked these two questions before going to the prep route for a year. HIs parents can afford the extra year easily, but he definitely did not expect a "no" from the academy when it came.
There are prep programs (NAPS et al) that have a higher success rate as feeders to SAs - no argument or challenge, but there are other prep programs that, well, do not necessarily match that. It's not a guarantee. Hope that helps.