I was able to comfortably make mistakes because I had no debts and significant excess cash flow every month from my job at the time.

https://www.har.com/content/newsroom/Houston Real Estate Highlights in May:-Single-family home sales fell 20.2 percent year-over-year, with 6,671 units sold, marking the second straight month of declines fueled by the COVID-19 pandemic;-The Days on Market (DOM) figure for single-family homes grew from 54 to 58 days;-Total property sales dropped 20.7 percent, with 7,917 units sold;-Total dollar volume dove 25.9 percent to $2.23 billion;-The single-family home average price fell 7.4 percent to $298,199, the first decline since January 2018;-The single-family home median price was statistically flat at $249,000;-Single-family homes months of inventory was at a 3.5-months supply, down from 4.1 months last May and below the national inventory level of 4.1 months;-Townhome/condominium sales dropped 36.0 percent, with the average price down 2.3 percent to $206,146 and the median price up 2.9 percent to $175,000;-Single-family home rentals jumped 11.9 percent with the average rent down 2.8 percent to $1,822;-Volume of townhome/condominium leases fell 4.6 percent with the average rent down 4.6 percent to $1,586.

What you want to charge are "fees" which is money you get to keep in your pocket.Some charge an up-front "pet fee" which is a flat fee the tenant pays for the privilege of having a pet.

If such a flat fee service does not exist, what are some things I should consider before investing my time in getting licensed?

It is a violation of federal law to accept rent from the tenant in excess of what is stated on the written lease. 

Buying the remainderman position in a life estate is a more typical arrangement...there was even a movie about a guy who bought one for a Paris flat..

After starting that job, I started saving money as fast as possible to house hack and renovate a 3 flat in Chicago.

The one I live in and another 2 flat which profits $900/month.I want to accelerate my real estate investing with the BRRRR Strategy but I am having a hard time calculating all the costs.

I pay 8% gross rents collected for property management, others pay $50-$100 flat per door .  

With my PM here in E TN we charge 10% flat with 50% first months rent also.