Scenario: you’re using opensource software and you aren’t donating yet, however you have enough money left over every month to do so. Something(s) is holding you back from donating. Think about what that is and what would have to change for you to change that.
It would be really hard to decide which open source projects would be deserving of such a foundation’s grant.
That said, if you want to support open source in a more general way, I would say you should donate to the Electronic Frontier Foundation (EFF). Cory Doctorow, the guy that invented the term “enshittification”, is part of EFF. I donate myself, it’s well worth it I think.