Can't say what's hardest, too many moving parts but take care of your tenants and build that goodwill, pays off in spades.

If you decide to revisit STRs, you might want to explore properties where you can project expenses independently, such as using local STR forums or online tools to estimate maintenance, management, and occupancy costs.For networking and strategy-building, you might find local meetups or groups beneficial.

I am hoping to be an asset to experienced investors, with the intention of learning as much as I can, so that I can start building a portfolio of my own.

He spent 20 years building it.

That's actually a meaningful silver lining.  

But that extra $7200/year will go a long way in paying down your loan faster, build up another downpayment fund, pay off other debt, etc. 

I'm working with a company who owns a small, 19 unit, senior and low income apartment building.

Additionally, you want to start building your "Core 4": agent, GC, lender, and PM. 

Any petdroppings must be picked up immediately, in or outside the building.

Meet fellow investors in your area and build relationships with them.