Yeah, I have a theraband flexbar and use it in my workout / maintenance routine regularly. It indeed works very well to strengthen those tendons... But the problem wasn't tennis elbow though. It was just general muscle sore, primarily in the forearm, and general rotator cuff issues. Rotator cuff is a weak spot for me. Had surgery ~15 years ago there for heavy calcification resulting in narrowing of the opening which was damaging my supraspinatus tendon, leading to pretty much permanent inflammation thereof. And as I wasn't playing at that point, and left tennis completely behind me, never bothered to train it back up properly again.
I was fine and pain free for everyday life stuff. When I started playing again with the 6.6 it as good as immediately flared up again... Had plenty of shots, physio and workout to strengthen and stabilize it again.
It's fine enough now, but I still have to be careful and make sure to allow the shoulder to rest after intense playing. If in a tournament I need to play matches day after day, I can't do it without starting up ibuprofen. Not ideal....
OTOH, had an MRI scan couple weeks ago and in it looked pretty fine, so there is much much improvement. Weirdly enough though, they saw signs of trauma on the humeral head which was somewhat consistent with trauma resulting from a dislocated shoulder. But that never happened... Other types of damage typically seen from dislocation also wasn't present. They asked me if I fell on it or if some object fell on my shoulder or something, but I can't remember any such event. If fell once at home a year or two, three ago and perhaps my shoulder did receive some type of shock then, but it's not like I had pain or something so didn't make any of it at the time... But it's the only instance I remember. Very weird. Anyhow, the hurt I have now in my shoulder at times, seems to be resulting from that trauma while the rotator cuff seems just fine
I'm playing masters championship this weekend and then the competition season is over. So I'm scheduled for another shot and physio after that to fix this last thing and hopefully then I can finally play tennis free of worry and pain afterwards

And THEN I'll break out the 6.6's again and hopefully not mess it all up again LOL