There are three ways to increase your resource cap.
1. Research (Which you have already done)
2. Build Sylvan Shrines: Each one increases resource cap by 1,500.
3. Elven Residences: Each one increase resource cap by 150. (Will increase past 90 population cap)
For the elven faction ~50k is a very decent resource cap. Hope I answered your questions. Let me know if you need anything else.
If I always appear prepared, it is because before entering on an undertaking, I have meditated for long and foreseen what may occur.
--Napoleon Bonaparte, 1831