The German tax laws for charities around having to spend donations in “a timely fashion“ are being loosened, but it’s really more a question of proper tax advice. It is a common myth in volunteer-run organizations than you cannot create long-term financial stability. You just have to lay out a plan and argue for it.

More of a problem is dependency on public funding or any kind of grant, where typically there are limitations on what you can use that grant money for, and “saving for the future“ is not one of them.