Im not sure if this fits your needs but i use kotatsu for mangas and they do have a few sources for comics. If you have the comcis downloaded then something like bubble2 or seeneva might be what you are looking for. If you are looking for sources to get comics then the megathread here or fmhy have multiple for downloading and reading them in your browser.

OP also didnt mention that they watch anime at all, i just included it to give them the option of obtaining german dubs if they happen to need them in the future.

I know, but it doesnt include german dubs

For german trackers there are non that are public, but a lot of private ones are really easy to get into. immortuos has a forum to apply for an invite and a one of the better general trackers but they do lack anime for that i would recommend anime datenbank tracker you can apply for an invite in their irc chat. There are of course more trackers but these 2 should be enough.

Edit: forgot to mention 2 things, prowlarr and jackett let you filter all trackers by language, this makes it easy to find new german trackers and see if you can apply for an invite or see if someone is willing to invite on the immortuos forum (its nor that active so it kight take some time).

For setting up the *arr suite someone linked this in an older thread here, idk how good that is but it might be worth a try.

I know of this userscrpit, worked fine for me the last time i came across something age restricted

Yes, you can just use the seedbox to download and run jellyfin locally with the media staying on your box, but it might be a lot easier to just get a seedbox that allows you to run jellyfin on it.