Ideally, the entire team should be onboarded before making a website accessible (product owner, developers, designers). 

It doesn't really work if there is only one person solely responsible for accessibility in an organization since that person will push against the goal of others to deliver fast. 

Make sure that it is a priority to train others during the work day to avoid people not prioritizing learning due to time.

A critical or primary user journey is a basic flow everybody has to perform in order to use the app.

(E.g. Add an item to the card)

A secondary user journey is a flow only power users can perform.

(E.g. Change your profile avatar)

Identify the user journeys (primary and secondary) of your product to identify the future work to be done on the most important product areas.

Then, apply an accessibility checklist to each step of each user journey. Run through the checklist with every team member.