You are in a great spot to start investing. Do you like what you do now? Keep doing it! Personally, I would keep my expenses low and buy real estate whenever I can. Target markets that might not cash flow as well but high appreciation areas. Remember, you don't need to leverage as much as possible, scale as fast as you can, and take a lot of risks trying to create as much cash flow as possible. Especially with where you are at. Just be consistent with accumulating positive cash flowing assets that will appreciate.
There's nothing wrong with a 15 year fixed investment loan. You will build equity QUICK. If you are struggling to find properties that cash flow positive you could look into longer options. Talk to other lenders, there are plenty out there! I got my first mortgage through my bank (never talked to anyone else) and later on realized how bad the terms were compared to other local lenders. You mentioned $500k equity in your house- if that house is worth $520k it's a lot lower risk to HELOC some of it vs. if you have $500k equity in a $3m home. Find a leverage ratio you are comfortable with and stick to it.
Our team works with a lot of out of state investors buying investment properties in Southcentral PA. Feel free to reach out if you ever want to chat a little further. Happy Investing!