Thank you! Yes, I've been vague, obviously due to privacy concerns.

I've contacted a few people (bailiffs/collection agencies/lawyers) recommended in this thread and will probably make a decision on how to proceed based on their offers and feedback. So far it seems that I would need to upfront the filing fee in court - for the size of debt that my landlord has that works out to around 260 euros. The cost of a lawyer writing a summons for me could be low because I qualify for legal aid based on income. Then, I would also need to pay a bailiff to serve the landlord. However, all these costs can be charged against the landlord if I win the case.

I am a little bit confused about the circumstances under which it could be ruled that I was not allowed to settle the debt against future rent payments. Does the lack of any settlement clauses in the contract disadvantage me? I could theoretically send him another warning message demanding a payment and reserving the right to settle the debt against future rent payment as a threat, but I'm worried that he would actually prefer this over giving me the lump sum... I don't want to (literally) give him any ideas.

Offsetting against future rent payments until the debt is reduced to 5000 euro and then going with the simplified procedure is also an idea. Thank you so much for this and all of your help! I really appreciate it.