I DID IT!! I GOT IT TO WORK!!!!I downloaded some smaller maps, but for some reason I just couldn't get them to patch correctly (wouldn't create the .dct file). The maps I tried were:
Western US and Canada 900.4608
Western US and Canada 900.4610 (auto installer)
And then I had a revelation - on the first map I tried (Western US and Canada 945.6243 auto installer), I was able to run FastActivate steps 1 and 2. But every time I tried to patch the navcore, FastActivate would always crash. If I re-opened FastActivate and tried to run step 3 again, it would say the navcore was already patched.
I realized that whenever FastActivate crashed, the ttsystem file would change from 6.03 mb to only 1 kb. My guess is because I didn't have enough "swap space" - with only 2 mb of free space left on the GPS, it just didn't have enough room to update and save the new file.
So I copied the 6.03 mb ttsystem file to my computer, put FastActivate into the same folder, and ran step 3 from there. Voila! Successfully patched without crashing! I deleted ttsystem from the GPS and copied the new ttsystem from my computer to the GPS... and now it works!
I'm posting my solution here in case someone else encounters the same problem. If it's helpful to even one person, that will be worth all the days I spent banging my head on the keyboard over this.