I’d say your work flow looks about right.
Fusion 360 is what I use. It’s free to hobbyists, though you have to go through menus. Pretty sure sketchup works too. Most CAD software has the capabilities to export as. .stl file, which is the current standard (but not only file type) used by slicers.
Here are some slicers. I personally like Cura, but I certainly haven’t tried everything.
IVI is going to be based on Marlin code by the sounds of it, which is great.
Answering 3 is really not easy. It all depends on the model and also the printer. My suggestion is pretty much to gain experience. Start with something like #Benchy which is kind of a stress test. This will probably not work out great the first few times, and will help determine weak esses in the printer. There are other benchmarking prints too. Start with something that is made to be printed without any supports. From my experiences with the few 3d printers I’ve used is they all have their own quirks. IVI will too.
One good way to learn before you ever get your hands on the printer is to watch some YouTube videos. I personally like Makers Muse. He articulates some of the finer points and has some good tutorials from care/maintenance up to 3d designing and printer settings.