Whether you want to stay with the same flavor or not, there is, truly, no safe, effective, way to clean a carto or any other delivery device short of a dry burn or rebuilding the coil over new wick. You really don't want to dry burn a carto, the poly fill will taste like crap once it's burnt. You could, I guess, take it apart, rebuild the coil/wick and stuff new poly fill into it, but that takes time and patience.
The problem is that, eventually, any coil becomes coated with a polymer like deposit that is a result of the carbon atoms in the PG, VG and flavors combining with the oxygen atoms also in them. It will build up much more quickly with sweeter flavors simply because there is more carbon in sweet flavors. The deposits are a pretty tough polymer and I seriously doubt that even an ultrasonic cleaner could break them up and any chemical strong enough to do the job is something you wouldn't want to inhale. Just for a laugh, I've tried lacquer thinner, paint thinner, acetone and brake cleaner. None of them touched that gunk on the coils so what it would actually take is a question I can't answer.
The bottom line is that compared to cigarettes, cartos are pretty inexpensive and can be discarded when their output or flavor drop off.
(To be read at double speed)
Just my opinion, worth what you're paying for it. Your results and mileage may vary. No warranty or guaranty is expressed or implied. Proceed at you own risk. I am not a chemist or physicist and do not play one on TV. The end user accepts all liabilities and hold the original poster totally harmless.