I can't upvote this enough. There are very few companies that are masters of this game. Because it is extremely difficult to build. Every stage is a painful pro

I can't upvote this enough. There are very few companies that are masters of this game. Because it is extremely difficult to build. Every stage is a painful process and there are so many moving parts. To begin with you need very high quality recordings. Successful companies have voice directors whose job is to record voice at most optimal settings (and to make sure speaker speaks is neutral dispassionate tone). And then there is the manual process of segmentation and phoneme alignment - That requires lot of human effort. Then you run the code! It will take quite sometime. Then you study the quality of synthesis. Fix issues manually (for example, if you are using concatenative synthesis, you may have to fix offending phoneme alignment based on feedback). And iterate on this. Unless you have a team, I wouldn't venture into this and rather use third party leaders.