Often your advisor in grad school will force you to focus on what they want rather than what you want to learn. This is less likely in a math department, but more likely in a CS one. As with everything, it all depends on your advisor.
The PhD is replete with hoops you have to take that will be orthogonal to your goal of learning (e.g. spending a lot of your time doing HW on the professor's pet topic when taking a course). Someone I know who retired somewhat young (early 50's) enrolled in a PhD program because he loves to learn. He dropped out within two years because he found it fairly inefficient in learning the topics he wanted to learn about (he had a career on mathematical topics and can handle the math). Unlike younger folks, time is precious for him, and being efficient is more important to someone in their 50s than in their 20s.
If all you care about is learning and not the actual piece of paper in the end, it may be more efficient to get a less demanding job and use your spare time studying what you want to study. Do it right and you'll make more money than you would as a student, and potentially learn more than you would in grad school.
Finally, PhD is about research. Yes, you will learn a lot, but learning is not the goal. A lot of people drop out because they realized they loved learning much more than doing research, which will involve large chunks of your time being unproductive. If you plan to do a PhD, you will have to draw a line at some point and say "OK, those 10-100 things there that really interest me? I have to drop them forever so I can do research." If you opt not to do research, you can learn a lot more.
Also, those in their 20s often haven't lived long enough to know how to gauge their efficiency. The person who has spent decades working 40 hour/week jobs knows better the value of less free time.
You are exactly the kind of person who should be doing it!
Good luck!
I am looking to also go on a self study mathematical journey
Introduction to Proofs:
Just pick one of these that speaks to you the most. All three are good.
Discrete Mathematics with Applications - Epp
Discrete Mathematics and Its Applications - Rosen
Mathematical Proofs: A Transition to Advanced Mathematics - Chartrand, et al.
Abstract Algebra:
How to Think about Abstract Algebra - Alcock
Abstract Algebra - Pinter
Abstract Algebra: A First Course - Saracino
Algebra - Artin
Abstract Algebra - Herstein
Abstract Algebra - Dummit & Foote
Linear Algebra:
Maybe an engineering based book first if you haven't seen linear algebra in a while (e.g. Strang or Linear Algebra: Step by Step by Singh).
Then:
Linear Algebra - Friedberg, et al
Linear Algebra Done Right - Axler
Linear Algebra - Hoffman & Kunze
Real Analysis:
How to Think About Analysis - Alcock
Understanding Analysis - Abbott
Tao's Analysis text
Principles of Mathematical Analysis - Rudin
Topology:
Topology - Munkres
Topology A Categorical Approach - Tai-Danae Bradley, Tyler Bryson, and John Terilla
Check out this list:
http://pi.math.cornell.edu/~hatcher/Other/topologybooks.pdf for others.
Category theory:
Categories and Toposes: Visualized and Explained - Southwell
Conceptual Mathematics: A First Introduction to Categories - Lawvere
Category Theory for Programmers - Milewski (if you like functional programming)
Programming with Categories - Fong, Milewski, Spivak (if you like functional programming)
Category Theory in Context - Riehl
There are a few others by Spivak which you may like.
If you don't know category theory whatsoever then I like Southwell the best (pair them up with his youtube videos). Eugenia Cheng also has a nice set of lecture videos.
If you already know math pretty well, then Riehl is a favorite.
Hope that helps!
I am willing to give up a high paying salary to return back to school as a PhD student and I am not tied down with a mortgage or anything else.
I find mathematics simply too interesting to not learn at the highest level. Working in industry will simply not teach me the material I want to learn. I’m considering going back for either a PhD in Math or a math heavy PhD in CS. I read math textbooks for fun and worked with tutors to ensure my proofs are done correctly. I can do this for hours on end without external motivation. I taught myself a lot of math and can see myself doing this as a career. I want to do research.
Most people my age say the same things in the comment sections in this thread (tied down to a mortgage, make too much money to return). I’m glad generalizations like these don’t apply to me and can’t wait to get back to school