Let's make some math:
2000000/8000 means it takes 250 days to refine, or 10 days to a 25 people fleet to refine (25 is the optimal fleet size according to cryptic).
The real challenge is to convince people to give 8k dil for 10 days to pay for the upgrade, which is completely stupid because everyone will try to be a free rider, not paying for stuff but using it extensively.
The solution is quite obvious, we need a daily tax on refined dilithium to fill fleet projects. It could be completely optional to avoid main complaints but of course only the fleets asking fora signifiant part of the daily dil would make progresses.