First off, I absolutely love your dedication and passion to this arena of DEI! With such a strong vision for the impact you want to make, I'm sure you'll be in a great place to take advantage of these opportunities you seek.
I recommend visiting your University's Office of National Scholarship Advisement. For Villanova, it's called the Center for Research and Fellowships: https://www1.villanova.edu/villanova/provost/crf/fellowship_opportunities.html. They recommend you make an appointment online and offer advisement on post-graduate fellowships. I would recommend making an appointment before the end of your junior year, as many post-graduate fellowship applications require a lot of work and you may need to begin working on them this summer.
I also recommend creating a free account on ProFellow, https://www.profellow.com/, where you can search their database of fellowship and graduate school opportunities.
Some of the programs I recommend include:
The Humanity in Action Fellowship: https://www.humanityinaction.org/fellowship/
While these aren't specific DEI fellowships, the Humanity in Action Fellowship allows you to design your own project that will address specific challenges in your community, so you could address DEI policies in an organization or institution within your area. It also comes with a year of professional and personal development opportunities that will only better prepare you to make an impact within your field. I have seen several AmeriCorps positions specifically looking for Diversity and Inclusion Advisors or Consultants that will help to improve these areas within a local nonprofit during a year-long position.
With regards to graduate assistantships, most programs that you apply for will award a select amount of students with Graduate Assistantships or Research Assistantships based on the quality of the application, and the award will be released with your funding decision. In many cases, you can also apply for specific positions once you begin the first year of your graduate program, during the summer before or the first few months of your program. There is not a guarantee you'll be paired with a professor working on a specific DEI project, but you can always include a statement about your specific research or teaching interests in your applications, and professors you'd like to work with, and schools will do their best to match your interests accordingly.
In addition, you can also research schools that specifically have opportunities for diversity, equity, and inclusion fellowships or have options to apply for such a program once admitted into your main course of study. For example:
Georgia Tech: https://diversity.gatech.edu/DIFellowsProgram
I hope this helps! Good luck with your future plans!