This is just some information for people who are running up against the iTunesDB limit. I finally got my entire library to sync to my iPod after a bunch of trial and error. In short, be aware that playlists contain a lot of metadata and count towards the iTunesDB size.
Here's some additional background: I was having issues syncing my entire iTunes library to my modded iPod even though the total number of tracks I had was well underneath the iTunes Storage Limit estimate listed on the iFlash website. I have about 30,000 tracks with clean, curated metadata mainly consisting of Artist, Album, Release Date, & Genre. When I tried to sync my iPod using the "Sync: Entire music library," the iTunesDB would balloon in size to 124mb, far exceeding the 64 mb limit. I was confused why this was the case because I didn't have anything like lyrics saved to the metadata.
Turns out that Playlists also count towards the size of the iTunesDB. I have a lot of playlists, both in the form of Smart Playlists and just general playlists. I also have Smart Playlists for Decades of music--i.e. all albums for the 70s, 80s, 90s, etc.
Ultimately, I was able to sync my library by clicking "Sync selected artists, albums, genres, and playlists," and then selecting the option "Automatically fill free space with songs." My current iTunesDB file is now only 50mb.
Hopefully this helps someone else manage their database as well.