Is there any other sponsoring LIR recommended? Thank you.
Joining gives you (in this case) also access to some "old-school" things like webspace, shell access, nntp, ftp, ... (similar to what for example SDF.org offers). They were originally there to help people getting access to the internet and building a website (think 90's internet), but they also do housing for individuals nowadays, and help educate people (locally in Berlin) on how the internet works, etc.
I have checked some other LIRs but they often only provide RIPE registration to their "customers" (what that means depends on the LIR but usually you have to have some hosting with them or have an ongoing project that they do for you, or something like that), so for individuals, I think non-profits or "Vereine" are probably the easiest to join
PS: yes, the website was a bit confusing for me too, basically you choose the cheapest offering ("IN BERLIN S" for 5€/month) and use the "Antrag für IN-Berlin S/M/L" from https://in-berlin.de/provider/antrag.html... However, as I understand it, they also require you to provide a "SEPA mandate" so that they can collect the fees directly from your bank account, and I think if you don't live in Germany that might prove difficult.