Don’t have any big recommendations for where to spawn units from, really. The game mode has some benefits in that it can only ever be owned by and exist on the server, that it cannot be destroyed and that you have access to it from anywhere. But as long as you are careful about which clients you give access to what actors I think any event called from the server in any graph should be fine. I’m by no means an expert on all things networking, though, but that is my answer based on what I know.
As for using the game state as a turn manager, that is actually precisely what I did when I first started experimenting with the big toolkit refactoring. I did this based on the suggestions I had, like you, read about it being a good place for handling the current turn etc. However, after a lot of back and forth I decided to use a separate blueprint for a number of reasons. The game state is mostly an extension of the game mode for stuff that all clients need to have access to, and there is a lot of stuff in the Turn Manager that is just for the server, so I felt it was a bit misplaced. Also, I wanted to have access to things like having a current unit marker controlled by the turn manager and running animate action events, which does not work in the game state. So a number of small reasons made me switch from using the game state to instead use a custom blueprint spawned by and referenced in the game state.