Seems you want to apply the DCA method but your means of achieving this seems to be risky. Like many have already advised here, centralized exchanges is not the right place to save your Bitcoin.

I understand that withdrawals can eat up significant portion of the investment if you have to be buying small amounts regularly and transferring to your private wallet. You can do it cumulatively to reduce the fees. You can deposit and buy then withdraw I two months interval or as you choose but never to save the coin in the CEX. 

Alternatively, you can use any of the numerous decentralized exchanges.

Once again, never save your funds where you do not control the provate key