I have my doubts that mapping left click to screenshot is a great idea. It renders mouse as pointing device useless and you may end as some Vim users: "I've been using Screenshot for about 2 years now, mostly because I can't figure out how to exit it."
To set folder where screenshots are saved:
- Press ⇧⌘5 or open Screenshot with Spotlight (⌘ Space; enter Screenshot in Spotlight search field).
- Select 'Options', and set 'Save to' as one of defaults or 'Other locations.
To take screenshot (full screen):
This takes screenshot of full screen and saves it to folder set.
I think that pressing three keys is simple enough solution. It takes advantage of the workflow built into macOS, no need for Automator scripts nor remapping.
For more information have a look at Apple documentation: Take a screenshot on your Mac