What triggers the site saving? Is it a timer, a hotkey, deselection?

It saved a few sites after a few seconds to more than just a few seconds but it kinda bothers me that I have no control over this

If you do nothing, it saves the site after a 10 second timer, to allow you time to add more deposits to the current site. This can be short-circuited by starting a new site of a different ore type; note that you can also cancel a site by tapping the ground, so be careful with that.

There are improvements in the plans for better feedback (e.g., actually showing the 10 second timer), and I'm also planning a complete redo -- I'm aware there are a lot of shortcomings to the current version, but it's been working (just barely) sufficiently for my needs (even worse, I've been away from Factorio for most of the run-up to 0.13, and even now I have less time than I'd like to work on it).

I guess I don't know what it's missing because I think it's great. I like the ability to add nearby deposits to it.

Couldn't even find a resource monitor for this to be "Yet another..." but I like it.

You can, if you click another within the 10 seconds it will include that one. So if you want to for all the fields at an outpost, just click all of them. The timer will reset to 10 seconds after each click.

I guess I don't know what it's missing [...]

I've got a large number of plans, most notably more kinds of filters than just "almost depleted" and "all".

Couldn't even find a resource monitor for this to be "Yet another..." [...]

The one for v0.11 was my inspiration, and I'm pretty sure it came from at least one other generation. For v0.12 there had been at least three other attempts at porting the old one, with varying degrees of multi-player instability. I keep expecting others to make a new generation, but apparently YARM is enough for now.

[...]if you click another [of the same kind of ore] within the 10 seconds it will include that one. [...]

Entirely correct, thank you.