Simpler option: sign up for the $25 (yearly) iTunes Match. This stores the music in the cloud (up to 25,000 non-iTunes songs, plus an unlimited number of iTunes music purchases), so it takes up essentially no space at all on the iPhone. Music can be put physically on the iPhone through playlists, or just streamed from the cloud (obviously you will want to weigh up how that might impact your data plan).
Or, if you prefer, a free option: don't try to load 100GB of music on your device. Simply load up a playlist of randomly-selected (or whatever) tracks that would take, let's say, a week of eight-hour-per-day listening to get through. When you do get through it, connect the iPhone to your Mac for say five minutes and refresh the randomly-selected playlist. Now it will load another week's worth of music and you're good for another week (or two ... or a month ... you get the idea).