"What does the public charity need to do in 2021 to maintain its public charity status"

There isn't a lot to go on, here. Did that $5m endow a school or fund a ball field or? Not all charities under 501(c)(3) get grants or fees from programs, or donations, for instance. It's a bit more variable than this.

"If the organization cannot meet the public support test for two consecutive years, it will be reclassified as a private foundation as of the start of the second consecutive year (unless it qualifies as another kind of public charity)."

From: https://www.irs.gov/charities-non-profits/charitable-organizations/exempt-organizations-annual-repor...

You can find other resources, such as:

https://www.nonprofitaccountingbasics.org/federal-tax-issues/schedule-key-maintaining-public-charity...

I like to work with the local United Way office; they are always current on the requirements, since they do a financial and governance review for anyone who submits for support through their network. They might be of help to you. And depending on where you are, you might find a Not For Profit Network, such as we have one that has brown-bag lunch training for this and other topics, throughout the year. Lastly, your State might have someone to help; I don't live in FL but helped someone there, and their Ag Dept works on NFP issues.