I've though about pumping it into a rain barrel to use it for watering plants
Excellent idea. The ppm is going to be very low on water from a sump pump. You can also use water from dehumidifiers.
slope- you have none ;)
Drainage class- maybe not poor for your particular site
Depth to restrictive feature- they're talking about bedrock here
Depth to water table- should actually be measured for your specific site
Frequency of flooding- self explanatory barring 100 year and 500 year floods
Frequency of ponding- another that is self explanatory
Available water capacity- another one that should be site specific
Some of that data helps determine areas in need of protection but there are other uses too.
I now know that I have Silt Loam in the first 15 inches and silt loam clay below that.
Your property has been "improved" because of the house that now exists on it. The above may have been what was there but that doesn't mean it's still there.
Go with what your little voice tells you to do when constructing your rain garden. You have good sound instincts and you're familiar with the basics. Fly with them. The only expert on that property is you. So what if you make a few mistakes here and there. We all do. Wanna dig a hole and only partially line it with roofing liner, go for it. Wanna go without roofing liner entirely, go for it. Unless you want to pay thousands of dollars getting professionals out there, let your little voice guide you.
The only true advice I can give you is to excavate deeper than what you think you need. Native plants generally have deep root systems. As a rule, I'm now going down 3' unless it's for a bog.