I used to mod SMK a lot, and MAKE is a million times better than epic edit for building the first draft of a course, because it has an automatic feature that will shape tiles around each other... Like, if I had generic grass tiles and generic road tiles placed together, there's the automatic brush feature that replaces the generic tiles with the appropriate grass / road border. so, instead of you for Mario Circuit having to click and drag each red/white road border tile, you can use MAKE to automatically build the road border around each road groove in seconds. It is literally that freaking fantastic. It's been so long since I've used MAKE, I can't remember which buttons do it.
MAKE will save you weeks on development time. You should seriously consider using MAKE for the first draft of a course, or if you need to redo some curves. You should be able to drag and drop an already developed course into the program to continue editing it. Epic Edit is much more clear on how to do the AI paths, starting routes, item placements, and for the last bit of tile fine tuning.
You should look up Youtube tutorials. It is slightly time consuming having to watch youtube videos and write down the keys to navigate the program, but it is absolutely worth it. I see you have your own tutorials on epic edit, so you must be really proficient in it, but you NEED to take a look at the MAKE automatic features.
To edit the road tiles in Epic Edit, just click the blue arrow pointing downward (below the tile map of the course) to download the sprite sheet onto your computer. You can use Paint.net to make any adjustments to the sprites. It was a small recommendation, I wouldn't mess with the sprites too much but if you wanted a Brown Bridge Barrier for DK Parkway, that would be an option.
For the 20 GP course comment, I just meant that you could have two (say) Royal Raceways, where you snipped different parts of the n64 map into 2 SMK maps. It's an idea, but if you already had plans, I would stay with it!