A lot of large companies are starting to gap the breach between manual test and dev. While part of this is writing automated tests for each feature, a large part of this is writing tools and frameworks to help with testing, debugging, and operations.
I've worked in a few "tools team" job descriptions. I personally enjoyed it. By being internal tools, it allows you to iterate faster and do more interesting technical things than could be publicly released. If you are working for an open source company, these tools can be very useful for other companies and are of great value when released.
Look for job descriptions that have tools, devtools, etc in the title, or any senior test automation roles where this would likely be most of your job (making the shared parts that everyone else uses).
Good luck!