What to spend $600 on for professional development (software engineer)?

Buy a ticket and plane ride to Poland for Lambda Days 2022. Depending on the flight and stay, it would cost close to $800, but you might be able to talk your boss into covering the difference.

https://www.lambdadays.org/lambdadays2022#register

I've considered finding an in-person conference for early-ish 2022. I don't want online confs, I just can't pay attention enough.

Is equipment allowed? Time to get a new chair, standing mat, desk or keyboard.

Maybe but probably not, based on how things are worded.

I’d posit that your best value for the dollar is going to be joining both IEEE Computer Society and ACM.

The membership benefits between the two will easily add up to $600 in value, plus you’ll be able to professionally participate in regional or local activities.

I will look into these, thank you!