Kinda depends how much practice time you have, the weather where you live, and the amount of dedication you can inject and still keep motivated to have fun playing tennis.
I worked on both my flat first and heavy topspin second serves at the same period of time. Flat's first, maybe 30, then a bit of rallying, then maybe 50 top/slice seconds, which can be used as first serves.
Gotta work on your backhands, as every decent player hits there all the time.
I played where year round tennis was normal...SanFrancisco....so don't know your court situation in the winters.
You can shadow swing all your strokes except volleys indoors, needing only clearance for the racket. Best to have a shadow or reflection to check your form while you swing.
You gotta practice everything, making your strengths stronger, and your weaknesses up to par.