Do you want a PhD or just to work in ML? Sounds like the former and if so just offer your services to a professor as a developer and forget about the degree.

Do you want a PhD or just to work in ML? Sounds like the former and if so just offer your services to a professor as a developer and forget about the degree. The fact that you ask the question shows you have no concept of what it means to do a PhD in CS let alone ML or any other field. It's nothing like a bachelor degree! So educate yourself a bit and approach some people who have Masters and PhD in History (your field) and see just what is required for higher level study (you'll be surprised it isn't as easy as you seem to think). Then actually speak to some people who have done a PhD in CS. Frankly, you're only 31 years old. Take the time to do it properly, no short cuts.