Lowu crossing is train only. So you should just take a bus or minibus to Choi Hung, switch to Kwun Tong Line to Kowloon Tong, then East Rail to Lowu.
To get to SZX, there's now a direct bus from Kowloon Tong MTR, via the Shenzhen Bay crossing. Other buses from Eastern Kowloon to SZX go through Huanggang, which I hate.