Couple of ideas.
1) sometimes tempo fluxuations can seem abrupt if there are not enough beat mapping markers. See if you can identify more sub beats to create a more gradual accelerando.
2) Assuming this is not a classical piece, there are probably sections of the song that repeat. Can you replace the part where the tempo begins to speed up with a similar section from earlier (or later) in the song. Sometimes creative editing does the trick.
3a) if you need to use the audio quantize, try quantizing only the sections you want versus the entire track.
3b) Sometimes the audio engine being used makes a huge difference. Acoustic guitar is one of those things where its very difficult to do without artifacts. I have alternate algorithms that I select, Speed from SoundToys can sound good. Also Izotope makes Radius.
bd