Hi there,
Greetings of the day!
I understand you would like to move away from O365 (SharePoint) to a Windows Desktop / App based application, and your are a business that is into Engineering & Consulting services.
More curious to know why you are planning to move away from O365 (SharePoint) platform. I believe it has all the requisites to manage your process automation you have noted. I have implemented and configured DMS for business verticals such as Real Estate Developer, Construction Companies, Consulting companies, and Retail. For few of the Customers, the DMS was implemented in SharePoint, couple in bespoke Web application, few in Dynamics 365 Sales, Marketing, Services & Projects. Happy to discuss your need to move off O365.
Since your ask is for a Window desktop / App based application, may I suggest you look at PowerApps (a O365 app) that allows you both Desktop and Mobile access using the same form. We could connect the existing SharePoint repository for data storage or look at Azure SQL or On-Premise SQL DB. We can also look at MS Flow (another O365 app) that can help us configure the workflows.
Of course, if you want to totally move away from O365, I can have it developed as a Desktop application with the App (Mobile) portion of it. It would be more scalable if you get this developed as a responsive Web app - that way, you can target the App customers without much change in code base.
Lets please connect to discuss further - no more space to write :) tx