I am one of the people you mention having had EC for over 20+ years of my life.
If I can suggest a few things:
1) get a GI test ; this will show what is going on in your gut, your immunology, digestion, absorbtion and microbiome. It will show if you have gut inflammation at which point you can look at potential causes of this inflammation.
2) have your doctor run a full blood count and have your thyroid and adrenals all tested. Have a full hormone test if you can. Test for b12 and d3 levels.
3) get lyme tested. You are fortunate to live in Germany; the best health care available in Europe.
I have done pretty much 1) and 2) and I'm waiting to have 3) done shortly.
It's important to have quantitative data so that you can formulate a plan. It makes no sense at all trying things without first knowing what is going on. For some reason when I have posted about tests here I get the odd poster saying it's a waste of money, yet the same people are willing to buy foods and loads of supplements without knowing whether they need them or not.
Like you, I attempted the leave alone method and it failed. It is very hard having EC for this long and I also have loads of other issues besides EC which compounds the problem.