Have half of BTC at hand. Want to find a way for better investment rotation to be done. So far invented such a scheme. The whole amount to be exchanged for any local currency. Surplus since last investment to be converted back to BTC. Remaining - initial investment - to be spent on daily needs. Converted BTC to be put back into wallet. Two points of interest here.
First: Does this way of turnover looks reasonable at all?
Got some question here, are you trying to day trade or long term investment with Bitcoin?
If you are into a long term investment then you must have your stable job. If you put all your funds or money in Bitcoin and then sell half of it for your daily needs, then eventually your Bitcoin fund will be depleted. Here is why, sometimes Bitcoin needs years to have a price spike especially when the investor enters in a transitioning market from bull to bear. Remember Bitcoin is not a get rich quick scheme. Sometimes you will find yourself selling at a lost if you do not have a stable job just to fill the budget for your daily needs.
In your situation where you have half the BTC as initial funding, it is a right decision if you convert all to local money if and only if the price of Bitcoin is at its peak ang the market is going down. Since you can always repurchase BTC even exceeding the initial BTC funding if the market price favors the buyer. It is a wise move if you fulfill the situation, selling at almost peak price then buying back when price goes down up to 50% during the bear market.
Second: How to make sure I use "clean" BTC from source of exchange?
If you are trading in reputable exchanges then you do not have to worry about your BTC being clean.