From what you say you sound to be rather stable and even independent of the Beta Blocker.... that suggest to me you are at very low risk of a stop, and it seems your doctor also sees it that was as you are still scheduled for tests.
I am not aware of any medications that would speed you HR up in a reliable way... you doctor surely knows.
I believe a pacemaker or an ICD may be needed to give your that extra protection against a heart stop.
There are others on this community who have specific experience, hope one of more will pitch in.
I echo the good info from Jerry,
The only other thing I can think of is identifying the source of the brady. If it's idiopathic and asymptomatic doing nothing is probably a good option.
If the brady is from av block or other conduction issue other decisions will have to be made including the possibility of a pacemaker to be on the safe side.
Your holter study should flush out the info.