If it were me I would try some aggressive pruning on maybe every other plant and see what happens
Lol. You sound like me. If something isn’t doing well I cut the crap out of it. Everyone screams “you’re killing it!” Then I literally give whatever it is a good thwaph and tell it to grow.
They always do. As the older man I used to call Skyfarmer would say, (he always looked into the sky before answering a plant question) “they’ll com’ on.”
But seriously, I do this to the sick,dying and healthy plants like a mad woman and have not had a serious problem yet despite warnings.
I will say you have to be careful SOMETIMES and not cut everything that needs to be cut at one time, but most of it’s just common sense.
Your elderberries? Don’t know a thing about them, but don’t think you would have the die back you fear by thinning. If I were very worried, I would take some and root them and then put in pots later. You probably won’t need them, but you can sell/give them away next year.
Just my .02. I’d read/watch YouTube and see what they say just because, but if you have that many plants, I can’t see the harm in trying different things. But I would maybe leave some untouched just because. (Probably not necessary though.)
Honestly? I think SummerThyme has hit it on the head.
Keep us posted. I always love a good plant mystery.