Also, can I continue training while tapering?
Don't understand, the BP and HR look good to me, unless you are dizzy at a HR of 53. Given your apparent condition it is not surprising you have such a low resting HR.
As for Beta Blockers, I have no problems doing radical does level changes. I have been on one or another beta blocker for about 15 years, as high as 100 mg twice a day. I'm now on 25 mg twice a day of Atenolol, my pick. I like its longer half life, as I have chosen to take a normal (not slow release) twice a day. If I miss a does I am totally unaware of it until I open my pill case and say "*)&^()" I forgot to take my pills (yes I take other stuff too).
I take BB and Calcium Channel blocker to slow my HR driven by AFib. So my HR goes up when I miss a BB, but it come back in line as soon as I get back on schedule.
When I was able to control my AFib with meds I was run for exercise guy, up to age 67, and found the meds slowed my HR only slightly. I have never had high BP problems, but have had unwanted very low BP with I was on the 200 mg a day BB, that is when CB was added and BB reduced, abruptly. I think my cardiologist sees no problem with sudden changes in does level. At the 25 mg level I think I could just stop taking cold turkey.. but that's me, not advice for you, people have different reactions.
Good fortune on your marathon - half is still way farther than any run/jog I have ever accomplished.