In the long term I want to continue to buy and hold rental properties to supplement my earned income.

What you are asking is should I be a real estate  speculator.Why do you want to be a landlord, do you know your state landlord tenant codes, will you have a property manager or manage yourself, do you have the skills to deal with tenants and evict, are you OK with tenants damaging the property, are you adverse to risk, will the property have positive cash flow or will you supplement your tenants rent monthly, do you even know what market rents are, is it your desire to own and operate a business, do you understand that equity in a rental will decrease the cash flow and possibly turn the investment into a liability, are you only doing this to speculate on appreciation, when the market turns in the future will you be ok losing money, will you hold long term when the market drops or take your losses and run ????????

We are planning to buy a few buy and holds to supplement our income over the next few years.

Selling could get you some cash on hand, while renting will help supplement your living cost.

Specifically, I am interested in the idea of supplementing my income as a software developer with income from note investing.

"This supplemental tax shall be referred to as the “CTA portion” of the Chicago Real Property Transfer Tax, and the $3.75 tax shall be referred to as the “City portion.”

I run a small business so my real estate investing is done more as a supplement than as an income.

It is a nice way to supplement that retirement check.   

There is a good podcast on BP that I heard that lays out a strategy of doing this once a year as a strategy as a supplement to your current income. 

Its a supplemental thing I do when evaluating a new tenant and I've found some good information on that site.Personally, get a property manager to handle it.