My advice is to make some offers and use your brother and the due diligence period to make your final decision. 

I have a SFH that I have rented out for 9 years. There is quite a bit of capital gain on it (about 500K). I am thinking about selling it now that I am about to retire (and the market is high). If I make my this rental...

Most Likely a scarcity mindset will set-in and it will impact your decision making processes on your investments. 

This being our first BRRRR, I would get overwhelmed at times and found it hard to make decisions on....everything, from paint colors to doorknobs.

You can bid higher than the asking price and make a lot of money immediately, but you have to understand what true cost to the REI is, and how to arrange the flow of money to and from the REI...through the deal first.3 - What a REI plan is, how to design one specific to the REI (not anyone else), and understand that your plan is a series of connected decisions, with each decision you make is based on the criteria needed to lead directly into the next step of your plan. 

I’m just not sure what the right decision is.

This is just anapproach to work through the decision.

When I finally reached $20k I approached my father and explained to him why investing in real estate would be a wise decision for me to build wealth.

You are make good financial decisions. 

I know tools such as the BP calculators are available for use, but I want to learn what goes on behind the scenes to fully understand the numbers (math) and specifically what it means so i can better gain an understanding & make better investment decisions when the time comes.Any advice/ tips or tricks on analyzing deals & double checking the numbers (before proceeding with the deal?