My biggest accomplishment was starting a fund as well. But I had a lot of others including a bunch of successful deal exits, starting a podcast, and releasing an online training course that had a lot of signups.
My biggest ah-ha was how when I take back a property the value is always lower than what I expect going in. I thought I was being very conservative already, but I will be adjusting my model for 2020. Borrowers who either sign off on a DIL or ride the ship down in a FC just seem to live very rough, even if the outside of the home looks good.
In 2018 I learned how not-passive note investing really is and how much attention needs to be paid to vendors. To address that in 2019 I brought on an assistant to help with that and it was a huge help. But as I head into 2020 I'm working on completely overhauling my systems and processes to make the business run better and be less of a 'hamster wheel'.
I agree not to be too quick to fire vendors, or if you do need to fire one do it in such a way that you have the option to return to them later if needed. One thing I have observed is that vendor performance can change over time. Sometimes a vendor goes to crap because there was an internal change or someone left, but then it gets straightened out over time. So vendor performance is not a constant. I think the key is to keep an eye on things and follow up when something isn't right. Just balance making sure things get addressed without being too much of a PITA.