Since you are capped at 30 seconds, I would work backwards from that, and arrive at how many filters to use to reach the cap. Sounds like, with the amount of filters you used, and keeping the ISO and aperture the same, you would have needed a lot longer of an exposure.
Given the filters you mentioned, which is 18 stops, if I have it right, you are looking at about 1024 seconds of exposure, more or less.