What structure or framework do you use for release notes and what's your reason and method for choosing that structure?

My advice to you would be to have the tech writer worry about release notes. PMM should do a quick edit before they go out, but this is a highly tactical distraction. You should be focusing on more important things. 

Our PM team has recently started a "mock press release" format for the original structure of release messaging, originally from Amazon. I personally think it's pretty interesting and the format forces you to think about what's news, why it matters, and how it'll ultimately be viewed by the customer/industry: https://www.rightattitudes.com/2016/06/17/amazon-mock-press-release/

I agree with Mike. Product marketing managers don't write release notes. They might clean them up and make sure we don't say anything stupid, but we shouldn't write them. 

If you're in an org where the PMM does have to craft the notes/announcements themselves, here's a structure that has worked for me:

• What we are announcing
• What it means to the customer. This section includes these elements:
• Main benefit
• Type of user who will benefit
• CTA
• Any action required on their part

I keep it simple, nothing too verbose. A well-written headline, a few lines of copy and an image will do the trick. Other tips:

• I try to position improvements (e.g. reduced crashes) in positive, rather than negative, language.
• My goal is to get the attention of the customer persona who is inundated with several other emails/notifications. Their mind is already pre-occupied with typical activities, phone calls, customers, etc. when they see my email. On top of this, I only have few seconds to grab their attention (assuming they see and open my email!). I then lean on the help documentation for more technical bits.

Agree completely with the other answers. 

However, at Oracle, when there was a minor or major release (not a software patch), we always created a What's New Deck that included bulleted items that were coming new in that release. This helped our sales and customer success teams to engage with prospects or existing customers. 

It is worth noting that we did not have a SaaS offering then, so these What's New Decks also helped when decision makers at our customer org were deciding on spending on upgrades.