The only thing I can say about this is that, in order to get excellent agents, I only have 10 agents, of whom two are backups. I undergo every ground mission possible, and I expend considerable air assets to create as many ground missions as possible. It's October (in the game) and I've got two Warrant Officers, I think 3 or 4 sergeants, and the rest corporals, except for the backups, who are still riflemen or maybe a Lance-Corporal. If the game were potentially endless (***hoping***), I could really put together a team of ultra-veterans whose fantastic stats would be fully earned. As it should be.
I beg the developers' forgiveness for saying that some training could improve agents' stats, although I think only hardened combat veterans should be able to get high stats. The problem with "training" in XCOM 3 was that one could get super-soldiers with nearly maxed stats who had never been on a mission, indeed faster than those agents doing the missions, because they spent time in hospital.
Rewarding only veterans with the highest stats would be in the spirit of the game, in which careful tactics and strategy conserve one's finest agents, and one is rewarded with agents who improve over time and can really slam the aliens. I find that I can keep my agents alive, but in order to do so, I honestly must carefully plan every single move of every single agent, every single turn. It's painstaking work, but highly satisfying to bring home the entire team, every time. Indeed, it's the perfectionist's game.