The manpower ressource is currently in a somewhat strange place to me. Basically, in almost all of my games I am either at 0 (early rushes, riftborn, vodyani), or at the empire limit.

I almost never find myself converting food to manpower via recruitment or buildings, since either I have a lot of food and am thus already drowning in manpower, or I do not, and thus can't afford to cut it even further.

Then there is the old problem that the only other way to spend manpower besides ground battles is the upgrade to tanks or planes, which costs a very limited amount exactly once. This means that usually you will switch to 100% tanks as soon as you have the T2 tech researched, and remain there for most of the game.

To make manpower an actually valuable ressource you have to manage and invest in, I would propose two changes:

• Eliminate "natural" food to manpower conversion. Instead, manpower is naturally generated in systems as a small percentage (5-10% maybe?) of system manpower capacity. This way, an empire that wants to generate a lot of manpower has to invest in its defenses, or use food to manpower conversion via recruitment or buildings.
• Tanks and planes cost manpower upkeep: You pay a percentage of your empire capacity per percentage of tanks/planes per turn, and a higher one to switch. Tanks currently dominate infantry, as they should, but at very little price, besides more expensive upgrades, and a small one-time switch cost. If the higher level of troops cost manpower upkeep, this would mean that only empires that heavily invest into manpower generation could afford a high-tech army; it would also give infantry, and racial bonuses/maluses on infantry, a much bigger role in the game as the "standard" army.