The economics of self publishing is interesting. When I think back on my first book I count at least 5 years research that enabled me to be able to write it, 3 years writing it and then many years selling the thing around industry. By a pessimistic account I was about £500k down :) Like you, the first year I sold about 500. Unlike online fulfilment today, I spent that time stuffing envelopes, staring at a slowly shrinking pile of boxes in the corner of my study, printing labels, with occasional trips to the post-office. It was actually work, and many times I thought 'this is crazy'. It doubled for two more years until dealing with the orders actually became too much (while doing a "real job").
On the other hand, it got me a string of well paying jobs, established me as the go-to consultant in a niche, invitations to talk and teach all around the world, and basically transformed my career. So putting a "net profit" number to it seems unfathomable. Each book you write, if researched from scratch, is kinda equivalent to a PhD.
What happened next is that I sold the title, from my own publishing company to a large publisher (MIT). Publishers don't take many risks these days, but they will buy a book that already has reasonable number of sales as a going concern. The advantage is that all mechanics and business is suddenly off your hands. Without an advance to pay back you then have a modest income stream and time to write the next book.
Another effect of being a semi-professional writer, as a side gig, is that other people want to pay you to edit, typeset, and review works. Learning LaTeX and various markups and text processing tools has been a huge gift. That can be quite fun if you're in a vibrant field that genuinely interests you. And you don't have to be associated with academia or a big-name institution for that to work. In fact I'd say that academics these days never have time for writing and that most other successful authors I know are independent.
I hope your book takes off even more, but right now I think you can count 600 sales in the first year as a definite success!