I have heart problems. I had heart failure and lyme carditis. The sooner you treat the better. It's good that the cardiologists is following you.
Welcome to MedHelp Lyme -- It sounds like your docs are alert and aware, and that can be the hardest part of finding the right path to getting properly diagnosed and treated.
I was diagnosed with Lyme and babesiosis (including an IGeneX Lyme test) and was a pretty sick puppy for a while there. I seem to recall also having a cardiac evaluation, but not an MRI or stress test, so can't speak to that, tho others here may.
(Note that there is no one single approach to testing, diagnosis and treatment in Lyme and its co-infections, so it is not unusual for each of us to have docs with different approaches. Docs don't always explain the why and wherefore of the tests they run, but if they know a patient is interested in understanding it all, they will usually explain -- as they should. Don't be shy about asking.)
Others here may have more experience than I with the approach your docs are taking -- and please keep us posted on how you do. All good wishes!