Sharepoint Online Code repository and continuous deployment

We are working on maintaining Sharepoint-Online themes, page layouts, styles (css, html, images from masterpage & siteassets) in git repository & continuous integration of changes to Dev, UAT & Prod environment. This is just the start of Sharepoint work. We will use VSTS for managing all these work. My question is:

1- We have some custom templates (html & css) specific to company & some templates provided by microsoft as part of sharepoint. We were planning to maintain only custom templates in git repository & not microsoft provided templates (because we are not going to change in m/s templates.). Can anyone please suggest whether this as per microsoft guidelines.? Is there any standard documentation provided by microsoft for organizing these files in source control ?

2- How to maintain testing environment for development changes ? Should we go for Sandbox approach or follow the approach mentioned in https://sharepointologic.blogspot.com/2017/01/sharepoint-online-continuous-delivery_17.html ? We have different site collection for Dev / UAT & Prod environment. If going through second approach then to verify every change, developer need push code to site collection. What is the best approach suggested by Microsoft ? I found a useful link but not able to decide : https://sharepoint.stackexchange.com/questions/82781/how-to-setup-development-and-uat-for-sharepoint-online-multitenant?newreg=cc9cc6db7e64476ca7b26e246f29b30b

Please suggest