AFAIK German law doesn't care how my filled out and double-sealed vote gets to the election office. Although I also don't see a point of why I'd hand it to some party representative when I can just drop it in a mailbox (I guess it might make sense as a service for less mobile people - who also can authorize someone to go fetch the voting materials for them), and I'd guess people just going round and ask to collect vote envelopes would cause lots of suspicion - door-to-door canvassing isn't really a thing here.

Are there widespread cases of fraudulent harvesting? Cases of harvesters telling people how to vote? Is there anything stopping Republicans and Libertarians from running ballot harvesting operations to turn out the vote in districts where they have popular support?

The most notable such case in recent US electoral history was North Carolina's 9th congressional district in 2018, where the fraudulent harvesting was perpetuated in favor of the Republican.

Why does there need to be empirical evidence of widespread cases of abuse before we address a measure that seems ripe for abuse? Was there widespread security breaches based on Spectre before operating system developers implemented mitigations?