Well yes, I know, I wouldn't have done it myself this way for many more reasons, but that's how I acquired the site and now I'm going through fixing and optimizing things. Amazingly is, that my tiny optimizations had already a huge impact on visitor stream, especially with users noticing them Alone bringing the Pagespeed from like 60% to >90% did increase user activity as also search results already by a 20-30% and revenue by 50% in less than 2 weeks! I can only suggest anyone to make a site speedy! Anyway... back to topic!
Well, in the case I would not have a page under /, I would of course had choicen either /de/ or /en/ to redirect to with 301 or 302, when visitors would have reached the home page.
But the more I think about it and after the thoughts I had previously on my post to Jane (which hers was also a great help, same as yours!) I think the best results might be really achieved by keeping the homepage as it is.
Just not sure yet, if I should add to the home page a x-default hreflang attribute or maybe none at all? Or maybe one for each language?
I thought even about doing a small test maybe first with other pages under the domain and protocol the results, before going after the home page, which is the one bringing in nearly all traffic.
Well, things seem now a lot more clear of what to do and what to avoid and I believe I can figure out a way now to make it work
Thanks everyone!