how can I move from shenzhen to Guangzhou and then back to SZ in the evenning

You can take G-trains or D-trains to reach GZ in about half an hour to an hour and a half t reach SZ.
If you want to take G-train, you need to go to SZ North Railway Station to take the G-trains to reach GZ South Railway Station. The G-train takes about 30 to 40 minutes.
If you want to take the D-train, you can go to the SZ Railway Station to take it to reach GZ East Railway Station. The D-train takes about an hour and a half on the way.