Where Does End-User Adoption Start?

Interesting question, I'd take a step back and look at what problems or issues the organization is facing, also what priorities are at the forefront and shape an adoption plan around that.  This means Office 365 is supporting the work people do and isn't just seen as something upper management is imposing on staff for no apparent reason.  

There are all sorts of things I could recommend, some of which you may be doing already, such as

build a champions program, so knowledge is spread across the organization.  These are your staff that  respond well to new technology and can kickstart and encourage other staff to try new ways of working

Build an adoption site, in SharePoint, that bring together resources, tips and guidance that helps staff make the most of these new features.  This can be done easily in SharePoint, like this - https://jaredmatfess.com/2017/07/16/sharepoint-communications-sites-for-your-office-365-adoption-site/.

Demos or webinars can be useful to help staff overcome some of the common stumbling blocks and help tackle issues upfront, these can be recorded and made available on-demand

Use reporting to find out more about how Office 365 is being used in the organization and what areas to focus on.  Office 365 Usage Analytics and things like Usage Score, will make this even more useful once available. 

Surveys - Use Microsoft Forms to gauge and benchmark departments on how Office 365 is seen or could be improved or better resourced.

Productivity library - For inspiration as well as lots of resources make sure to check out the https://productivitylibrary.fasttrack.microsoft.com/en-us/office/envision/productivitylibrary, which can be easily filtered on industry, roles or products.

https://fto365dev.blob.core.windows.net/media/Default/DocResources/en-us/Microsoft%20365%20User%20Adoption%20Guide.pdf - Has a complete framework for adopting Office 365 mapped around common scenarios, that tie in with many of the resources in the Productivity library.   

Plus you could do things like have a fair, roadshow etc. promotional events that give staff a taster of some of the top features that will help them be more productive. 

Hope that helps a bit.