You can publish it on google play now, as it is... and if you later decide so, edit the storyline, add a level or two, and sell it on PC later.
The advantage is that a) you get some money now, and b) when the final version is ready, you will already have a few fans, which will be more likely to buy it. (Another advantage is that if your game has some bugs or other problems, you can use the feedback to polish the game before you start charging players. I suspect a paying customer will be more angry about bugs.)
From what you say, it sounds like it would be quite a while before ad revenue from a free game would pay back what I spent on commissioning 3D artists.
An ad banner like in AdMob would interfere with gameplay quite a lot. The control scheme is designed for full keyboard (but would work well with a game controller with joysticks). It would take significant work to translate it to a tablet screen (a cell phone screen is definitely too small). Maybe this kind of annoyance would be a feature if I was trying to sell a full version that was ad-free alongside it, ...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.