(Located in Germany) Finally got the dreaded lawfirm Letter demanding 900€ for a John Wick torrent I accidentally left running. best course of action?

I got one of those years ago and never replied. Never heard back again.

A friend got one some time after that. He got scared and got a lawyer. He phrased a response letter with a reduced payment. He explained to me, that the phrasing is very important. You cannot under any circumstance show guilt for the alleged infringement, because then they come back with a vengeance (demands in the thousands of €). Paying the 900€ would already equate an admission of guilt.

1. chill. they will give you a short time to respond, usually just one week. ignore that, its bullshit. they will send another letter every week, increasing their demands. if they send letters with tracking, refuse to accept them, so they return to sender. they cannot prove that you got these letters.
2. ignore these idiots as long as possible. the only thing you MUST respond to is the big red letter from court, where they try to get a "title" over you, which means you MUST pay. reject their request 100%. pay them nothing.
3. maybe, if you are polite, send them a "cease and desist" letter, in german "unterlassungserklärung", where you promise to NEVER upload that content again. but not the original version! use the version from https://www.abmahnwahn-dreipage.de/ ("modifizierte unterlassungserklärung"). use a regular letter, no need for tracking. they actually have a right to go to court for this, but they rarely do, because court is expensive.

sadly, too many germans are stupid, so they pay the "fine" (its not a fine, its just a false demand by another citizen, zivilforderung). often its children who do the filesharing, and its the stupid parents who pay the fine and then hate their kids... or these lawyers love to send these letters for filesharing of porn, in the hope of destroying the relationship between husband and wife.

this should be basic knowledge for everyone in germany, then this shit would stop, and we could simply use torrents instead of stupid OCH/usenet.

its also funny that they demand repair of damages (schadensersatz) for content which is propaganda, causing mental damage or waste of time. see also: out of shadows (2020).