It is unlikely you will get a quick answer if you post your questions to the champion program group instead of the product group. Read this: Getting the most of the community when you have a Marketo question
Yes, the "update" date will correspond to a "data value changed" activity log.
And yes, your approach makes sense, although you can also use the get paging token endpoint to start at a specific date the retrieval of the activities.
Thanks, Grégoire. That's helpful. Thanks also for your advice about the group I should post my question.