I'm not sure I understand your post. You have heard of BiggerPockets but then are asking for generic advice. They have blog posts and guides on almost any topic you can think of. Have you read the Ultimate Beginner's Guide https://www.biggerpockets.com/guides. Coach Carson https://www.coachcarson.com/ is another great site.
But all of your questions are almost impossible to answer without knowing details. Some people specialize in only short term rentals, others do just fine with long term. Some people enjoy being more hands-on and being their own property manager, others like to have one less item on their plate. There's multiple paths to real estate, but without knowing more details impossible to give you more helpful advice.
That, plus, or rephrase:
Everyone finds their own niche and works in it. I can have the similar to you financial situation, live in the same place as you do, but do totally different RE investment than you because of my low/high risk tolerance. I, for example don't feel comfortable over-leveraging my properties, that's why I use OPM (CC to finance my properties renos short term) and then own them free and clear. This and the fact that my own salary is low, makes it impossible for me to get a loan for my rentals. So, I use cash and CC.
I also don't trust other people to take as good care or do as good of a job as I do (that's in relation to you property management companies question). I have a long established relationship with some professionals, like a plumber or electrician with the Masters license who make sure the things are up to code and pass codes for me. But I don't bother them with service calls that I can handle myself. And since I have built my properties from the studs up, only I can troubleshoot everything effectively, because only I know what is behind the walls. No management company has the same interests as me: preserving my property long term with the least amount of spending (or a reasonable spending).
Question #4:
No one can tell you that, it's pertinent to your financial situation. Yes you put down 20%, but you might not even qualify for refinancing or another mortgage. I have more than $200k of equity in my primary residence, but don't qualify to take even $80k out. Every time I log into my account to pay my mortgage, I get bombarded with "use your [huge equity] to go on a dream vacation," but I've tried (to pull the equity out) and I don't qualify. I don't spend money I have to borrow on a vacation. So, here is the simple answer to #4, talk to your banker. If you qualify, compare the monthly payments, the closing costs, the taxes and insurance to see how much you can spend on a property and how much you have to rent it for. You might get to a conclusion that stock market is the king.
I live in a great neighborhood, but when I look at how much people buy properties for and how much these properties are rented for (and that amount does not include the renovation costs and the maintenance), I struggle to figure out how the .... do they make any money? They can't just loose money for the sole purpose of shielding their income from taxes? Or could they? And that's why I don't buy my rentals in my neighborhood, my math doesn't work here.
Airbnb. I did short term rental of the lower level in my personal property. I had a special agreement with our largest teaching hospital and was hosting visiting students. A few years ago, I could rent it for $850/ month. Now the competition is so high, I dropped the price to $600/m and still not very busy. Below $600/m I don't really see a point in continuing doing that, more headache than reward. I totally don't envy Airbnb people with the turnover every week or a weekend. There is one location I like to visit myself on Airbnb with a private lake and a huge fish, but again this years there were a lot of people fishing and it isn't as private as it was a few years ago.
Again. it's just me. Someone might find that $600/m isn't much of a headache. But building a property from the studs is.
So, ask yourself these questions: why do you need a rental? Why not stock market? If the money wasn't an issue, would you be doing rentals? What "edge" do you have to beat your competition during the purchase and over the tenants? These are the main questions to ask. These are your motivators. 20% down or refi your house are just minor details that could be easily solved, if you really want to get involved in RE.