There are a lot of good resources here on BIgger Pockets and few other reputable resources as well that go in depth in all these areas.As you pursue the path of a real estate investor, ask yourself questions like do you want to be an active investor or passive investor?  

I'm seeking a referral for a reputable, responsive property manager in Jacksonville, FL.I just bought a 2-unit and would like to buy another 1 to 4-unit prop in the next 2 months.

https://www.barrattassetmanagement.com/I have no affiliation with them but they have a great reputation here in Indy.

We have been active members here on BiggerPockets for the better part of 10 years and have earned a reputation for impeccable integrity and never shying away when we fail to hit our standards. 

In this industry reputation is very important.  4.

Is there a way to know if an online real estate license course is reputable/accredited/accepted? 

What you think you'll save, you will spend in purchasing.Find an honest, reputable builder that will work hand and hand with you and in the end, you'll be a happy man.Luke 14:28  For which of you, desiring to build a tower, does not first sit down and count the cost, whether he has enough to complete it?

But I've been dealing with a reputable real estate agent that is telling me that a cap rate of 5% is not unreasonable...

Start immediately with the 8% who might not have the best service reputation but they're in your affordability range.Whatever you do, be honest with your expectations. 

The biggest turnkey company of repute only works in two or three markets.