How often do you meet with product, sales, and marketing to discuss customer adoption and what do those meetings look like?

Those meetings must be (1) cross-functional and (2) on regular basis which makes sense for the rhythm of the business.

1) Cross-functional means that reviews should include all different functions/departments relevant for the topic, consider Product, Support, Marketing, Sales, Sales Engineering, Customer Success, Services/Consulting. This way the meetings are actionable and end with committed action plans.

2) The "regular" cadence would depend on the number of customers you need to service and how often you interact with them. I wouldn't consider anything less than a monthly meeting and in many cases a weekly or once every two weeks is a good frequency.

The agenda can be divided in two parts - Hot customers and In-focus customers. "Hot" refers to customers which for some reason would need regular review of progress and adapting to changes, e.g. new big customers, challenges with adoption, big ongoing issues with the service/product, escalation of sort, etc.
The in-focus customers are all other customers in your portfolio, which have not been reviewed in those cross-functional meetings for more than 6 months (this duration can be changed to whatever makes sense for your business). The idea is that each and every customer is reviewed at least once or twice a year, assuring the whole organization knows what can be done to improve or maintain their experience and what could get us to the next level.

The review of individual customers may include - usage patterns, key challenges, support history, customer satisfaction feedback, recent QBR findings, changes in the customer's strategy, changes in the environment of the customer which may impact their direction and adoption, new use cases and features which can help adoption.

If you start with those meetings, make sure that:
1) The purpose of the meeting is formalizing and committing an action plan for each customer and that means the meeting ends with such plans. Don't meet in order to decide that you will meet separately to make a plan. This must be clear to everyone and it is best to have it prominently on the agenda and meeting invitation.
2) Asynchronous preparation and follow up - all key roles must come prepared to the meeting. Materials required for decisions are provided ahead of time for everyone to read up on them asynchronously. Certain roles and a chair of the meeting make sure that regular follow up on previously agreed action plans is done asynchronously in a tracking document, so that everyone who wants/needs to see the updates can do so outside of the meeting. This allows the meeting to be focused entirely on its purpose - see (1) above.