Are you near a major university with a concentration in the field you're interested in?

If I were you, given that you have a (theoretically) nice, high paying job, I'd just contact the department and inquire about auditing a graduate seminar for a semester. That will let you get your fix, without having to give up your job. If they want you to throw them a few grand, as a lawyer, you can probably afford it. It's the best of both world, really.

You can get a lot out of books (and journals, don't forget the journals!), that's for sure, but as u/iLXGTtypeR correctly points out, reading books alone will not give you the access to people and resources that come with being a part of a history department. And that sort of thing really matters. You'll be able to bounce ideas off of colleagues, have in depth discussions about works in a way that's not really possible anywhere else, including online.

If your goal is to become a professor of history, that's a good reason to get a PhD in history. (Although be aware that there are fewer and fewer tenure-track / middle class professor jobs nowadays. Colleges rely more on adjunct professors, who they pay a few thousand per course, rather than paying them a middle class wage.)

If you just enjoy studying history, you can do that on your own without getting an expensive degree.

I know what you mean, in terms of liking the school atmosphere and that being more engaging than work... but if you don't like being a lawyer, a better course of action would be to think about what field you would like to work in, and how you can pivot towards that. Don't get a PhD in history just because it would be interesting to study, you should also think about what would happen after the degree.

it's not worth going into debt for- so if you can get fully funded then why not? There are many things you will get from an intensive PhD program that you can't get from reading books- access to historians, developing writing skills, places like conferences to test out your ideas where you can get (most importantly) criticism about your work. However, it is also super stressful and there are very few academic jobs in history. BUT you can also use your PhD to do other research jobs, for example, a few of my friends with history PhD's work in high paying jobs at corporations, and for the government doing "research" jobs.

PhD student here. It is pretty much impossible to pursue this degree with the vigor and intensity it requires AND work the 60-80 hours a week you're probably pulling as a young lawyer. In addition to the reading and writing you'll be doing, you'll probably want to try getting an assistanceship/TA position from your school, so that you can start accruing teaching experience and building a CV. If you're not planning on a career change in the near future, there's no real reason to enroll in a PhD program.

Notice how I said enroll in a program. Most grad programs I know of will accept a few non-matriculated students, if for no other reason than that their tuition money will go to pay for some else's TA stipend. Depending on the program, you might even find a few faculty members that get REALLY excited by the prospect of someone wanting to study legal history. If you think you might want to take the plunge into academia after taking a few classes, you might be able to apply those credits to your degree. I'd recommend speaking to at least the department chair of any program you're considering looking into, and preferably one or two professors on top of that. You have every reason on earth not to do this, so if you're going to spend your time in a classroom at night with a bunch of idealist fighting off an impending sense of doom/futility, you want to make sure that you're going to have the best experience possible under a professor whose passion and interests you respect. If you don't think you're guaranteed that, then maybe look at another school or put the whole project off for a few years.

If you're serious about a career in academia, it's required. That doesn't mean you need to get it immediately, though. I know guys in their late 30's-early 40's who are working on theirs.

Disclaimer: Not a PhD myself, just happen to know a bunch of academics.