Tips for the real estate hobbyist:
Manager your own properties.
Try to do as much of the rehab work as possible yourself, based on your competence.
Attempt to make any repairs yourself before paying somebody unless you know for a fact you won't know how to do it. HVAC is something I never mess with, but I will put my hands on pretty much anything else.
Try to create relationships with reputable vendors. This is almost impossible for me because I only have two rentals and they don't require repairs very often. I did get a discount for having 3 properties' HVAC systems serviced at the same time recently (included my own house). 55 vs 79 bucks for a tune up and 15% discount on any parts if needed- not bad.
Have tenants deposit rent directly to a bank account.
Have mortgages auto debit from those bank accounts.
Buy your rental stuff at Home Depot with a dedicated credit or debit card. The register remembers the card and HD will email your receipt to you. I just learned this. Have not noticed that Lowes is doing this yet. Use HD or Lowes coupons at HD whenever you have them.
I don't think I am at the level of "professional" yet, so I understand your "hobbyist" mentality. I think you want to try to automate as much stuff as possible, while still be willing to do manual labor for stuff.