How will an unrelated PhD affect my job chances?

Typically your compensation is based on relevant experiences and achievements. The key word here being "relevant". Degrees and experiences outside your field of work don't add much to your paycheck unless you find a way to make them relevant for your job. So it would certainly advisable to pick your humanities research topics with an eye potential intersections with what your career plans are.

You often get a few brownie points for "self discipline" and "proof of being able to work independently" if you manage a Ph.D. while being employed in parallel. On the downside, you will be competing with people in the same age group that have more direct work experience.

in the long-run, will my ROI in pursuing a Master's in an unrelated field be positive or negative?

Impossible to know. This depends on a lot of details. If you are applying to an engineering job, a degree in the humanities could actually be a detractor (since many engineers have low regard for "the other side of the campus"). If you are working in behavioral economics and your degree is a credible source of behavioral knowledge, this might help.

Overall a Master's is a significant risk if it comes at the expense of direct work experience and achievements. Once you have a Bachelor, in most fields direct experience beats academics hands down, even if the academics are in the same field. Unrelated fields makes this even worse.

And what if I go further with a PhD in that field too?

There are a few fields were a PhD can be useful (life sciences, medical) but other than that it's mainly for academic careers. I have a PhD in the field I work in, but PhD in that field are not that common and most people I work with have no clue that I have one, nor would they care. It's been years that anyone has called be "Doktor" and then it's typically just by people who want to sell you something.

Another one of my rules: Don't get an advanced degree if you have to pay for it. Not getting a free ride means that the university doesn't think you are particularly good at this and they are primarily interested in your money, not your brain.

From an employer's perspective I've been in the situation a couple of times where I had an interesting candidate for a job but they also have an unrelated PhD or specialization which wasn't really a merit.

It really does matter a lot if the candidate got a salary claim matching their background or if they are willing to ignore that merit entirely.

In one situation the candidate was fine with ignoring it at first and got hired, then after some years quit because of salary. In another situation a candidate turned down the job because it would mean a significant salary decrease compared to what they were used to.

So when searching a job where you PhD or similar deeply specialized knowledge isn't a merit, be up front in the job application and state that you don't expect it to be a merit in terms of salary. Otherwise employers might get cold feet when reading the CV and dismissing it.

I don't have a degree in anything, but I have a senior engineering role in an IT capacity and have worked in the field for almost 30 years. Education wise, I'm actually pursuing something of a personal interest that has nothing to do with IT. I have room to do it now, as my children are grown. I feel like I'm thriving at work and doors are still opening, and my point is that none of this is tied to having a formal education.

Education can give you a lot of facts about a lot of things. But the bottom line in a business setting is that no one is going to just pay you for being able to regurgitate a lot of facts. This would apply just about anywhere. What's ultimately going to put money in your pocket is your able to synthesize Idea A and Idea B into some advantage that creates money, opportunity, stability, improved process, or some tangible asset for your employer. The degree doesn't earn your employer any thing. The application does.

Your resume should be a list of doodads and whats-its that you have managed, improved, shipped, recalled. Measurable results, which implies your own ability to measure them and then communicate them. The commodity could be anything. With the utmost respect to your education efforts (it's tough!), I'll say that this is what increases your salary and job prospects, and not those little letters behind your name on a certificate.

Good luck.

So my question is: in the long-run, will my ROI in pursuing a Master's in an unrelated field be positive or negative? And what if I go further with a PhD in that field too?

Negative. The key concept here is "opportunity cost". Every hour that you put into your unrelated field is an hour that you are not putting into your related field. If you get a MS degree you might put in, say, 10 hours per week over 2 year, maybe more, so 1000 hours. For a Ph. D. it might be 4000 hours, maybe more. The opportunity cost of 4000 hours is pretty big. You could learn a whole lot about a related field in 4000 hours.

Now if your life's dream is to get an academic job, if you want to become a professor or researcher, if you want to switch to this new field after you get your degree, then it could be worth it. But don't do it just for fun.

Others have given a few (valid) negative points. But I believe it's not the whole story.

You mention in the description that the company is American, but tagged Europe/Germany as keywords instead, and that can make a difference. Setting aside the purer reasons such as personal fulfillment for extra studies, there are definitely cases where it would also help your career there.

1. For the case of the Master's, it does happen that some higher roles require a Master's degree; even regardless if the actual field is relevant. In Europe, graduating with a Bachelor's alone is considered normal for practically oriented professions (say, to become a nurse or middle school teacher). But after finishing a theoretically oriented Bachelor's at a proper university, continuing with a Master's is kind of expected (and quitting after the Bachelor's alone can be even frowned upon). The Master's is not only meant to certify specialized knowledge, but indicates being able to think and operate independently at a higher level, which may be needed for leadership roles.

2. A PhD is a different story. I have a science PhD which may be a little different one from a humanities one, but it is quite challenging so may be harder to do part-time while working elsewhere (note that often you can get paid a stipend to pursue a PhD, but the amount can vary a lot depending on the place). Likely, the topic of your PhD will be irrelevant to future employers. If you do want to upscale your profile for finance, it's going to be tricky but may work if you are able to pick up specific research skills that are relevant to them (eg for market research, data science to get more into quantitative finance...) and communication skills (presenting at conferences).

3. Likely, further studies will expand your horizons to some extent in ways beyond what can be written down on a cv. For example, a better understanding of history could allow you to think more strategically about your own life and career.

Good luck whatever you decide!