Congrats on the new spin in the hobby!
For the substrate question, there can be several answers and some depends on what you want. Do you want a pretty uniform look for the bottom? Or do you want it to look more natural? I rarely find a spot in a river or lake where the bottom is totally one type/color. At the beach , yes, but other places, no. So that leaves liking a mix of things on the bottom. I mix the stuff as I like it better and it is also easy to do. The plants don't seem to care as long as they get fed. Over the years, most of my tanks have a very assorted mix of Flourite, gravel and several types of sand. It all gets mixed and thrown around as the currents and fish see fit. To me, natural but to others really lame. Personal choice?
Part of the decision for me may be that I do not try to keep the tougher plants to grow. I go for the fish and then choose the plants that will work with the fish. Fish first, plants if they work. If not they are replaced with those that do.