This is really fact-specific (interpretations of EU261/2004 compensation rules change over time in various courts pretty frequently). A service like EUclaim or one of their many clones (at least one is a YCombinator alum) might do the work for you in exchange for a percentage of any compensation they manage to squeeze out.
The angle would be something like this: it is not Lufthansa’s fault that there are thunderstorms in Frankfurt, but if they do not adequately staff reserve crew at their Frankfurt hub, then their inability to operate flights after irrops might be their fault.
The steps are something like (1) send a strongly worded letter to LH saying that you’re planning to litigate this and citing the reasons you think you’ll win; (2) litigate it. You can litigate this yourself or hire a service to do it (there are many such services).
A pro will have lots of cases at hand which are relevant to your specific circumstance. The case of whether or not EU261/2004 compensation is owed after a flight was cancelled due to crew timing out after a weather delay was addressed in, for example, Case C-294/10 ( Andrejs Eglītis and Edvards Ratnieks v. Latvijas Republikas Ekonomikas ministrija ). The court issued a very unclear ruling in which it said, look, an airline must plan adequate crew schedules and reserve crew to handle some occasional delays, because occasional delays do happen sometimes, but the carrier must only act reasonably and must not make intolerable sacrifices. See e.g. https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:62010CJ0294 linked from https://www.flyertalk.com/forum/air-france-klm-other-partners-flying-blue/1690361-flight-cancelled-last-night-would-i-eligible-compensation-eu261.html in a similar situation.
It’s kind of fuzzy, right? Which is why it’s an area where you could litigate or hire someone to litigate for you. Your expected value from following up is almost certainly positive.