It would be good to know if you come across any advice for this (e.g. specialised job boards, etc.)

So far I've managed to avoid jobs I consider to be less ethical (e.g. developing proprietary software, high frequency trading, etc.)

Now I'm wondering how I can do something that's a net positive, rather than just keeping the cogs turning in some indifferent corporate machine...

I might take issue with some of what they consider high-impact (e.g. imagining how many friendly AGIs can dance in a pin-sized server) but it's a place to start at least.

The most obvious way is to think of an idea that helps fight climate change but also makes money (easier said than done).

Also been considering if there is the potential to gain suppose on Patreon or similar from the community for working on climate action projects like some open-source developers do.

You could try to contribute to that or get hired to work on that project or something similar.