Hello
Thanks for posting a clear definition of your project. In reading the requirements it sounds like there is a strong need to synchronize offline/online data. This is common in sales, emergency management, and other roles where users aren't always online.
Therefore it made me think of "SharePoint Workspaces." This technology comes with the ability to synchronize offline/online data. SharePoint also provides the standard CRUD and data storage that your project describes. So, for example, a list could be created to store the customer details. This list would have Excel edit capability, online edit, offline edit, bulk import/export, security permissions, and other functionality that comes with SharePoint.
My solution does not match your project specification (PHP/SQLite) so it would be best if we could discuss this first. I can provide more insight if you'd like to learn more - sample SharePoint sites, architecture diagram, etc. The SharePoint based solution would not require any custom coding and would be an off-the-shelf product that you can configure and manage into the future.
I am located in Alberta, Canada and have worked with many clients across the globe. I am a native English speaking freelancer and can bring creativity, dedication, and a wealth of experience in the computing industry to your project.
Thank you for your consideration.
Jay