Support candidates who have climate change mitigation as part of their top priorities. Local, state, federal, whatever. You will have no greater leverage than through public policy. Until an election cycle occurs, aggressively engage with your representatives or pay an org to do so.

The US military is one of the largest CO2 emitters in the world. No non profit or startup can fix something like that, but public policy can. At the local level, you can form a muni electric coop and buy only renewable energy in bulk. You can change zoning to phase out gas stations, disincentive internal combustion use, and acquire electric buses for mass transit (both cheaper and faster to deploy than light rail). Similar changes can be made at the state level. But these changes are then impactful act scale.

Find a political point of leverage and exert maximum force. I understand the squeamish feeling, but these are not tech problems; these are people problems. The tech is already mostly solved.

I do agree that answers in the vein of "doing more stuff" doesn't help when someone wants to spend money, not time. But with a mindset of incrementalism, vegetarianism and veganism can be mostly effortless. Mentally/spiritually grappling with and letting go of the social incentives and punishments around having children (called "natalism" by some) may take effort, but if you are already in a position where these don't really affect you, it's effortless: it's not doing something.

What's more, I can frame purchases within these lifestyle choices what OP wants: buying more expensive substitutes for animal-based food I'm used to is investing in the environment and the well-being of sentient creatures. Purchases for hobbies, adopted pets, and experiences is money I would have spent on child-rearing.

Right.

I wholly agree with all of those. In fact I'm doing all of the ones that are practical for me.

The main purpose of my post is - I believe that my talents in software development allow me to use that for good.

Essentially, that by me voluntarily reducing my consumption and redirecting it towards climate efforts, I can do more and faster.

I think this is genuinely more efficient than quitting my job and going all-in on protest or working directly for a charity.

Whether it's more efficient than starting one - time will tell.

You miss the point of condoms. They do not work just for you, and a sizable chunk of the world population has limited or no affordable access to contraceptives, or doesn't know how to use them. And there are quite a few charities dedicated to changing this, thus not only helping the climate, but lowering human suffering (starvation, overcrowding, STDs) in general.