You are right that your response was not optimal, and therefore it was a the right thing to switch to a new TKI.
The thing is with the guidelines is that they don't really take into account what should happen when you switch. You can't expect results instantly, and nor can you really reset the clock so some middle ground interpretation is needed.
From your results, you have gone from 4.9% to 1.7% in a pretty short time - a reduction of around 2/3rds. That's not a bad trend and your next result may be much lower. At one point on dasatnib I went from 3.9% to 0.5% in the space of a month. Progress isn't always linear and in fact rarely is.
Did you have time between nilotinib and dasatinib where you took no TKI? When I switched from imatinib to daastinib I had a couple of weeks treatment-free and my PCR went up a lot in that time.
The 0.1% goal by 12 months is optimal and you, like me, did not achieve that. That doesn't mean you won't ever get a great response (I'm happy where I am, now). I'm not entirely certain how to re-measure what is an optimal / warning once you switch TKIs. Perhaps someone else here has a better idea?
With regard to cytogenetics, as I understand it a complete response is correlated with a BCR-Abl ratio of 1.5% or lower. But this can vary in any one individual.
From the doctors I have spoken to they have stressed that the destination is the important thing, not the journey. If you are a slow responder, but are responding then that really is OK. Some people on this board took years to get to MMR (0.1%) and are doing just fine many years later. I know it's stressful and we all want low results, and we want them yesterday but if I were you I would take comfort that the trend for you is lower results - things are going the right way.