I think the cap for minerals needs to be 750 per planet and the research cap needs to increase by 150 or so for each level. that would be the easiest and simplest way, just increase the caps that way.

Just make the cap on all resources about 12 hours worth of production and each resource structure add an extra 1 hour worth of storage space.

The people who log into the game regularly will still have an advantage over those who don't, and will make the structures worth buying for players of any level/rank.