First published on TECHNET on May 08, 2018
Did this research for a customer and wanted to put it here for you and myself...
As we had discussed to be able to create a custom Site Collection template there are a few steps you need to consider after you perform ‘Save as Template’. As well I have added a second method for consideration.
Create your Template by ‘Save as Template’, then edit the solution so that you can change it from Sandbox solution to a Farm solution.
Instead of creating separate templates for different functionalities you can create an add-in to run and add the desired objects ie List or WebPart. I recommend you start with this
to understand the basics, then you can start digging into the articles below.