Ok, OP is super analytical - it works, it has made him successful.
Also, I applaud your confidence in reselling every 2 years in this housing market. I honestly love stuff like this, and love reading about it.
Since you are really focused on efficiency, it makes sense that you would think over practicing will get you to the place you want to be. It won't work that way. You have to factor in recovery time in sports or you will get hurt and derail everything.
Past that - without a doubt buy a stringing machine.If at all possible do not get a drop weight. Since you are smart about money, let me drop a few facts in :
1. No string job will last as long as you want. The closest is natural gut. So either get natural gut or read on.
2. The cost of getting a racquet strung is $20-$30, so let's call it $25. At the rate you play, you need to have 2 sticks ready. I would string one a week with the hybrid setup you use so that way you are rotating in a fresh stick and the old one can be an emergency backup until it is restrung. That will cost you $100 a month in stringing fees.
3. If you buy a reel of OGSM for $40 and a reel of Black Magic for $95ish your string beds will cost you less than $5.
4. If you buy a good stringing machine for $500 you will pay for it in less than 6 months, and your total cost for tennis stringing will be $20 a month.
5. Im summation, following my system you will always have a fresh stringbed every week and your total cost for stringing will be under $30 a month after the initial investment of a string machine and 2 reels.
Hope that helps.