Metric would be perfect if 10 wasn't such a dog shit number to base our counting off of. Sure it works for dividing things in half, but how often do you need to break something down into fifths? Halves, thirds, and quarters are 90% of typical division people do, with tenths being most of the rest since 10 is that only number that our base system actually works with.

Yup. The reason I went with yards was because I knew 1760 was closer to a nice multiple of 3 than 5280 (neither 5200 or 5300 is a multiple of 3; I'd have to go to 5100 or 5400).

But yeah, imperial works pretty well for multiplication and division, it's just not intuitive for figuring out the next denomination. Why is a mile 1760 yards instead of 1000 or 1200? Why is it 5280 feet instead of 6000? Why is a cup 8 oz instead of 6 (nicer factors) or 10? Why is a pound 16 oz instead of 8 oz like a cup would be (or are pints the "proper" larger unit for an oz)?

The system makes no sense as a tiered system, but it does make calculations a bit cleaner since there's usually a whole number or reasonable fraction for common divisions. Base 10 sucks for that, but at least it's intuitive.

I think my 7yo visualizes the number line in their head when there's no paper around, but they draw it out in school. I personally don't understand that method, because I always learned to do it like this:

 7372
+ 273
=====

And add by columns. With a number line you add by places, so left to right (starting at 7372, jump 2 hundreds, 7 tens, and 3 ones), whereas with the above method, you'd go right to left, carrying as you go. The number line method gets you close to the number faster (so decent for mental estimates), but it requires counting at the end. The column method is harder for mental math, but it's a lot closer to multiplication, so it's good to get practice (IMO) with keeping intermediate calculations in your head.

I think it's nonsense because it doesn't scale to other types of math very well.