RE: Can D365 Customer Insights be customized?
You can definitely ingest your D365 data into CI directly through Power Query/PPDF using the DataVerse connector (since 365 sales data is there). So you can connect it, but you're ingesting it, running it through Map/Match/Merge and then exporting it out. The Customer Card would just let you show the combined history of all the contacts for instance, in a card so it shows all activities on a timeline. But you have to ingest the data and then export it back to CRM although I think that experience and integration will get stronger soon.
So you can definitely ingest the data and export it back - what I meant was that you can't just change the customer card like you can with a form/view in D365. You know how you can edit or create Forms, and have multiple views, right now that's not a feature in CI.
You mentioned advanced Analytics though - so depending on what those are the answer is it depends. CI has AI feautures like Product Recommender, Customer LifeTime value and a few more. You can segment and export those or use them to feed PBI dashboards. There's also website analytics that you can see in real time if you activate Engagement Insights, which is still a new feature but very easy to setup and quite cool.
As far as CI though, the analytics you get are mainly going to be the models and then segments/measures you build off of the activities.