I left another career (military, rather than tech though) to go back and do a PhD. I didn't really find any barriers to it - I just applied like anyone else. I basically cold-emailed some professors doing things I was interested in and went from there.
> There are areas I feel I could be helpful with, e.g. creating reproducible experiments or ensuring a codebase could be open sourced in relation to a paper. It's a lot of grunt work that nobody really has time to do.
Whoah caution there. Don't go into a PhD with the idea that you can be helpful with someone else's work, or doing grunt work. Doing a PhD is very personal - you need your own big idea to work on. You may be part of a lab, your work may even be one part on a larger project, but you must carve out your own very distinct part of that and generate your own research. You can't really do a PhD by helping along with something else. Open sourcing a code base is not a research contribution.
You could make a PhD about how to make science more reproducible, though. You'd need some big idea about how to do that.
It sounds like you might be interested in looking at being a research engineer as well. Prestigious places like Microsoft Research hire quite a few of these I believe.
https://rse.ac.uk