GiveWell does some of this work of evaluating charities, but if everyone was donating to GiveWell instead of some to other charities I am sure those other causes would suffer quite some.

Following the Kantian maxim isn't good in this case. In effective altruism there's a concept called "room for funding". If you would have 10 billion $ and seek a target, GiveWell wouldn't be able to use that money as effectively as it uses a marginal dollar at the moment.

At the present moment simply going with GiveWell is a good option if you don't want to spend much time. They provide you with proven courses that can put your money to good use.

It's also possible that you see something in your community that would be done if there funding but nobody stepped up to pay the bill. Paying for a entry on meetup.com for a local effective altruism group might be an example of that category.