Any tips for establishing a security champions program for the software team? If you’ve done this, did you run into any internal pushback or skepticism?

Executive support and buy in. It helps to have clearly defined roles.

We have established a Product Security champion for the entire company first. This person for us sits in IT within the Infrastructure Security and Risk Management group.
Then every division has their champions and sponsors. Since my team is very large, I have appointed two champions to create a backup. This was done by asking my leaders including lead architects for nominations.
The team finds it very easy to communicate via our champions to the Company Champion.

Initially it looked like asking the IT's security champion or working via him was very cumbersome but now after a couple years, it seems great to have a non-partisan person help make decisions. They helped create a SOP on how we include Security in the product (Secure Product development Process). With them in a driver seat , it helped get it done and everyone does it. They also are responsible for Security toolsets. He has monthly meetings to review the dashboards, convey anything new he is hearing.
Quarterly the sponsors are invited to share the individual division dashboard.