How do you work with stakeholders to prioritize ongoing enhancements?

I have several clients with sites and platforms that have fallen into disrepair over the months and years due to a lack of ongoing enhancements, improvements, and iteration generally.

I'm looking for methods or processes to better manage these clients' priorities to make sure that, after we clean things up and make some improvements, we don't end up right back here 9-12 months down the road.

I am considering a quarterly or bi-annual workshop to discuss the state of the platform, review user data, do some user interviews or observations etc. but this seems maybe a bit heavy.

What are some ways you have worked with clients or internal stakeholders to uncover and priorities ongoing enhancements to an existing platform?