I don’t think scaling by military points is necessary. If you have military points, you can choose the option to get more free units.I think I would prefer that. It's not really fair that in some cases you can bring forward like 20 units, and in other cases you only get like 6 or 8.
The other option would be that the number of units you get are scaled based on your military points. So you get like 8 + 2x the number of military legacies you complete. If you go 3/3, then you get up to 14 units. If you add up over the course of the game, that might also help the modern era - if you only went 2/3 in the ancient era and 2/3 in the exploration, then you have 4 points of units to start the modern era. I wouldn't even hate if the other legacies had similar default bonuses. The attribute points are nice, but sometimes you do get to the age transition and it like "I spend all that effort, and all I get is +2 happiness on the capital"
If you want a reward for investment, just make the order reversed..cavalry, infantry, ranged