Thank you so much for your help and insights! I really appreciate it!

Not being relaxed may be a big concern. I´ll try to keep that in mind, next time I´m smashing.
And beside that I spend around 10min on leg (I thought, I heard Kelly recommending it in one of his videos), which may have been too much.

Regarding flossing: I use an old bike innertube. Will that be sufficient? And how often would you recommend doing it?

Are there some other ways to get the stiffness out of my quads? E.g. couch stretches? And if so, what can I do if I have problems getting my torso upright?

Thanks again!