Thank you for your detailed answer :)
Savotta sleeping mat sounds good, i guess you meant sth like that, i think id like this one cause it already has the underlaying rainmat included
https://www.amazon.de/dp/B07DPST4YW/?coliid=I3LMMGTRURB28L&colid=2D6PMAU4MPBZ8&psc=1&ref_=lv_ov_lig_dp_it
For the tarp i guess ill go with a budget option, my dad also told me he still has his army poncho which can be used as a tarp, i guess ill just try this one first if its still waterproof :)
Sleeping bag sounds a bit difficult, i thought maybe in the summer one of those (very thin, but non-waterproof) bags might be ok, so probably i can focus on the spring and fall in terms of sleeping bag.
But the modular option you mentioned sounds very interesting too, especially in summer it might be a good idea to have a protection against insects with the bivy bag. I definetly have to check those non-fully-waterproof ones, cause the rainproof ones i saw yet were extremely expensive.
Before i thought about the Carinthia Defence 4 sleeping bag, but this one is quite expensive too.
Backpack will get a bit difficult yep, i really like those Bagpacks with those attachement straps(right word? im not a native english speaker as you might probably have noticed :D) for additional sidebags, but those mostly are very pricey.
but i will definetly follow your suggestion to buy the bag at last, that totally makes sense, and i didnt see that before actually :)