Looking against the European Leukaemia Net Guidelines, to obtain an "optimal" response at 3 months your PRC should be lower than 10%, and at 6 months you would need to be below 1%. At 12 months this would need to be lower than 0.1%. So, your response is not optimal. That doesn't mean it's been bad, as the trend of results is downwards.
So if your response is not "optimal", what is it? Well the ELNet guidelines have three levels. Optimal, warning and failure. Your results fit into warning at 3 months (>10%), warning at 6 months (1-10%) and again warning at 12 & 15 months (0.1-1%).
However, you results for the most-part are at low the end of the "warning" range.
So there are two arguments here. Be proactive, get out of the warning zone and into the optimal one ASAP - so switch to another TKI. Or, be more patient and take solace in the fact your results are going down and consider you just may be a slightly slow responser. Your tolerance to imatinib might play into that ... if you tolerate it really well otherwise, and get no side effects, that may influence the decision.
Easy to say from a distance, but I wouldn't get too concerned. Your doctor is trying to get you from an OK response to a great one.Your results have been nowhere near the "failure" thresholds in those guidelines.