Long-term stock investment spanning multiple generations

I don't know about other jurisdictions or Germany, but in the UK the solution to this problem would typically be to set up a "discretionary trust", sometimes called a "bloodline trust" for this sort of application. Some more details can be found here https://unite-wills.co.uk/guides/bloodline-wills-and-trusts or https://www.aprilking.co.uk/making-a-will/bloodline-trust-will/ for example, and googling the phrase will turn up many more.

Of course it still suffers from the problem of needing to choose some trusted person(s) or institution to act as trustees and correctly implement the wishes of the trust settlor (the person who puts assets into the trust).

And that "rule against perpetuities" mentioned in Flux's comment on your question would also seem to be a potential obstacle - as this gov.uk page points out "there rules to stop trusts in England & Wales from existing for an excessive time"; however the the current "perpetuity period" (from some 2010 legislation) seems to be 125 years which would hopefully be enough for your purposes.

UK trust law has a quirky history originating with 12th-century crusaders needing to leave their assets in the hands of a trusted party before heading off on a journey from which return was uncertain, so I'm not sure what equivalents exist in other countries, if anything.