No, when I designed the bottom part of the rocket I had not yet researched larger joint decouplers, so I went with like mk1 decouplers
Anyway Thanks for your feedback, I already use struts (lots of struts), however I don't have autostruts and I have no idea how to check my TWR, and yes, a part of my upper stage is 4 mk1 tanks in a row
So meanwhile I gathered answers I had a second look on available engines suitable for my rocket and replaced all my engines with vector engines and replaced the entire lower area with just vector engines and some larger fuel tanks https://imgur.com/a/Cyo2ZWk (second image)
I also added a shield at the top to move all the drag towards the back of the vessel <3
Flies ok now (a little heavy during launch, starting at like 5 m/s, but at least it files)
Also all my answers have to be moderated so it takes a while for my messages to reach the forum page