A little background:
I'm a data scientist and machine learning entrepreneur. I co-founded a sorta successful company in the AI space that got top-tier VC investment. Certainly not a unicorn or household name but a solid business that has been going for over 6 years and has a realistic shot of getting acquired by an industry player at some point. After many years, I've stepped down from my operational role and I'm working on other stuff.
I have an MS, I didn't get anything published, but I worked on computational biology, heavy on statistical methods. I found the atmosphere focused on lab seniority which emphasized pursuing publications to set up future positions in academia sort of empty, so I graduated and started doing my own thing. While I felt disappointed in myself at the time, it ended up being a great decision that resulted in developing a love for building businesses.
The Problem:
I want to go back to my local University and get a math PhD. There are professors there who have great expertise in fields adjacent to my research interests (theories of statistical learning).
I got denied last year and I want to apply again, but I will likely get rejected again. The feedback that I'm getting is (1) that I don't have relevant letters of recommendation from academics (they are colleagues I have worked with), and (2) the lack of any substantial math focus in my undergraduate degree.
I can see where they are coming from -- because I didn't publish, they aren't really aware of my statistical work. It wasn't a class on a transcript, it was doing work for my PI. My undergrad was in biochem, at that time I wasn't even really into math, I took the classes I had to take. Grad school was over 13 years ago; people who knew me well are unreachable or dead. People who didn't work with me that closely aren't even sure we worked in the same lab.
Outreach to professors is understandably not very effective. Even though they think my background is great and my research interests are on point, they don't feel comfortable writing a Rec for someone they don't even know. I don't blame them.
But now I'm "locked" out of academia. I know some great young students who got accepted -- they are really just starting out! They are training and building models for the first time ever. They are outstanding young researchers, but they are just really getting their sea legs. They were shocked to find out I can't even get into the program as a PhD candidate, knowing that some graduates have interviewed at my company!
So, for the next application, I'm going to take the Math Subject GRE. The GRE is waived because I have an "advanced degree" but I think it will go a long way. Letters of Rec are trickier - I have been using professional references. Beyond that: what else can professionals like me do to avoid being sidelined for PhD admissions?