We dont have or use SharePoint and I dont want to use our IT Department. So SharePoint Online is the obvious choice, but I want to build a software engineering team to not only build a Web Enterprise Portal but also use it to access other Data-sources at our clients Datacenters, and want to proceed down a path of Custom Development (.NET) with full Mobile Visualization with new custom web pages, apps from within the Web Portal using SharePoint. How do I decide? Online (easy) or On-prem (expensive). Will SharePoint Online 2016-2019 give me everything I need? Would SharePoint (on-premises) but totally in the Cloud (Azure) be another choice?
I think Online should theoretically suffice, since it has all the necessary interfaces, but the performance is very, very bad and contrary to many claims by Microsoft, your network is probably not the cause. It simply is their servers taking their sweet time (tested different networks and locations, CDNs cannot help, since static content is not the issue).
If you are fine with waiting about 6-12 seconds for a page to load, you could use SP Online. We regret our move to the cloud world a bit because of the obvious performance problems. Cannot say anything about the performance of azure-hosted SharePoint.
It is a great advantage to not having to administrate SharePoint servers on the other hand.
I would recommend to get a license for SP Online and try it out. You will probably notice immediately what I meant with "performance problems".