your EF is very low, no wonder that you feel tired. As you have been told, the pacemaker is a recorder as well, so the cardiologist can see, what happened in the past. For example, it may show a lot of A-Fib's. 40 % of the pacemaker-recipients show them. Normally you don't feel them. Cardiologists have a lot of possibilities to adapt the pacemaker to your need. That takes time. You did not mention your additional medication.
I would recommend you to ask your cardiologist, if a medication with digitoxin (contraction of the heart muscle) could improve your EF. It is a old medication, but now there is a clinical study (Germany), whether the digitoxin can reduce the admissions of the number of patients with heart failure in the emergency of a hospital. As I already posted , the treatment with digitoxin ( I have a pacemaker also), had a positive effect on my heart function.