Do you guys know of any trustable websites to buy Play Store gift cards from? And also, is there any other way to buy apps from play store using bitcoin?

Hi, I couldn't find anything about it on the internet so i decided to start this.
I've been mining on a GTX 1060 for the last 5 months, when the when the profitability was at 1 mBTC/d (Using Nicehash), and BTC price was ~800$. After the price started rising my profit increased to about 1.9 mBTC. But now it's is at 0.8 mBTC.

I'm still kind of noob, so here are my questions:
What was the reason there was an increase in the profit when bitcoin price rose?
What is happening to the profit now?
Is there hopes of the profit rising again? Or at least stay at this and don't decline more?
I was planing to get seriously into it and buy a couple of gpus. should I go for RX 470 (provided i can find any) or GTX 1070, or should i wait for Vega?
Thanks.