<<you have to laboriously click through the slow, confusing miasma of PHP sadness that is the Steam partner site and add them one by one.>>
Read question: Can this be automated with Selenium? For a long time I was using very low level Chrome DevTools libraries. Then I switched to Selenium. It is so impressive. It actually works, without weird 1-5% failures that you waste time chasing... then adding small sleeps to make it smooth and working.
I would suggest playwright or puppeteer myself. It uses the dev communication channel for chrome(ium)/firefox and works much better, even if you're limited to JS... there's a Deno port of puppeteer if you want to avoid some of the node.js bits for (imho) a cleaner experience.
Read question: Can this be automated with Selenium? For a long time I was using very low level Chrome DevTools libraries. Then I switched to Selenium. It is so impressive. It actually works, without weird 1-5% failures that you waste time chasing... then adding small sleeps to make it smooth and working.