If you invested $250,000 cash then that would be how you calculate your ROI. If any of that $250k is part of the line of credit then it's not your cash in cash return.
I think you really need to figure out what YOUR goals are and what you want to do in the future in order to get the right answer to your question. The community needs A LOT more information to even begin giving half decent advice.
Also, you mentioned the full time employee. What another poster was saying was you should try to calculate what the cost to renovate a home is. Then you can add all other expenses like carrying costs and cost to acquire to have a "total cost". Then if you were to sell you'll have an accurate profit. Otherwise for example you could buy a home for $50k, buy $10k worth of materials and assume your total cost was $60k. But if you paid your employee $20k for labor to renovate then sold the home for $75k you would be losing money. This is an extreme example but I'm trying to illustrate why you calculate expenses this way so you can have a true picture of what a home cost you.
Do you need the cash flow or do you need a lump sum of cash? The way I decide whether to hold a property or whether to flip it is to determine what the income would be on a yearly basis. For example $6000 a year. If I could sell it for a $60,000 profit, it would take me roughly 10 years to earn the same $60,000 if I held it. Of course I'm not accounting for principal reduction but I also wouldn't have to deal with tenants and management and repairs etc etc.
So for this property how much will it cash flow each month? How much will your profit of you sell?
Also, it appears your doing a great job. Id do a little more research or read some books on the numbers side of the business. For example each rental you listed has its own expenses. They are all probably different. If you lined up all your rentals you would see, according to the 80/20 rule, that you are actually earning 80% of your income for only 20% of your properties. Seems extreme, it's not. You might even be losing money each year on some properties and not know it. My point is you sure have a clear picture of how each property performs on a monthly basis and you'll be able to make better decisions on what to do with them. It would be pretty easy to sell a property if it's losing money right?
With all that said, I would just keep doing what your doing if all your numbers are accurate. Your doing something right. Not too many people on this site have been able to find that many deals at such rock bottom prices. I will say that we are at or at least bearing the top of the market in terms of price. In other words I think you can't go wrong by selling right now. Whether interest rates go up, or supply increases, or Trump does something crazy I feel a market correction looming in the next 12 months if not sooner.