However, the profit I am making on SF ($300-$550 a month) could be made on each door of a MF/apartment complex.

I visited the property and confirmed that it is a profitable opportunity.

If there is some larger projects that are not a part of your $20,000 rehab budget, then thats probably right. 

I invest in RE to grow my seed money quickly, and to take the returns in the form of cash, so I can move forward even faster.I take the profits, and use some of it to invest in stocks...but I treat the stock market like a bank account. 

I know investing in real estate is never completely passive, but from those of you with experience in larger properties like this, what types of situations typically come up that the owner has to resolve or become involved in?

Look tirelessly for a deal until you find one so perfect and profitable that you cannot help but jump into it.

Here some interesting info and numbers about the project.Acquired: Feb 2018Rehab - 2 - 3 months.Listed - May 2018Sold - Sep 2018Purchased - $115,000Financed - $110kRehab - $63k (investor contributoin.Sale Price: $224,900 with seller contribution $6kAfter all expenses Final Profit - $21k  - 33% Highlights - Removed big pool and created big back yard, did major foundation repair to get to tolerance level, removed walls to make the kitchen open, all new flooring, new roof, new appliances and whole shebang. 

That will allow you to A). make some mistakes with a lot more leeway than a newer investor starting now or have them work as investment properties at your basis where someone buying today would be cash flow negative or B) eventually sell or refinance the properties to raise the equity for higher return or larger scale investments.   

Sell, Sell, Sell.Now is the time to off load all SFHs and enjoy the profits.

To that point we began to look at larger properties in the 50-150 unit range.