Correct! It's replaced with CG as of "Conundrum" I think
I thought so. I've always gotten a sense they go episode by episode, and improve as they went along. The drawback is that after the improvement is made, they are either unwilling or incapable of going back and fixing the older work, likely due to time constraints. Examples of that are season 1 where certain model shots weren't available for Encounter at Farpoint, so CGI was used instead. But then the shots were found and used in later episodes within the same season, yet the early CGI remains.