Signalling that people need to compare charities is going to first cause people to withdraw before it causes them to re-invest.

You have many good points overall, but I'm not at all sure that we have a real alternative to "signaling that you need to compare charities". This is perhaps the most critical part of effective giving, and trying to avoid it just makes it look like you're pushing your pet cause with no good reasoning behind it. (I mean, just look at the debate within the EA movement. Should you care more about saving lives, promoting animal welfare or averting existential risk from newly-created AIs? It's a big mess - and the very reason for this is that comparison is no longer helpful here).

Even if the perception that donating effectively is "too hard" makes some people withdraw, that probably doesn't matter much due to how comparatively ineffective most charity donations are.