See below for some video lessons from In2pickle. The lead instructor for this site is Tony Roig and he is very detailed. He takes actual situations and analyzes what they did, what they should/could have done, etc.
https://in2pickle.com/pages/pickleball-videos
As for the slice return of serve, since you have this type of shot in your arsenal, you can use it on many returns without too much effort. Again the purpose is not to win the point on the return, but to get to the kitchen line much easier.
As for slice serves, I use them to some extent and more so if the wind is moving left to right when serving from the "ad court".
As you move up in level though, you will find most agile (even if older) players can get to these serves and thus if your "out" serves are more than 1 per game, it probably is not worth it.