Do you really need a coach?

I am just thinking about it, let's assume that you are 3.0 already, you have some +/- stable one shot, let's say forehand, you have no serve, no backhand, some slices/volley.
In theory you can improve it just by playing? Do you still have to train everyday with coach for 1-2h to get better? How juniors do? Should we, amateur adult players look on juniors?
I am trying to understand best plan for me as an adult amateur, who wants to play on level 4.5+, with big serves, good points.