No standard abbreviation exists for nautical miles but definitely don’t use nm because newton metres

Since as you mentioned Newtons are N not n, Newton meters are Nm. nm means nanometer.

The knot is non-SI but perfectly metric and actually makes sense as a nautical mile is exactly one degree meridian

I do admire the nautical mile for being based on something which has proven to be continually relevant (maritime navigation) as well as being brought forward to new, related fields (aeronautical navigation). And I am aware that it was redefined in SI units, so there's no incompatibility. I'm mostly poking fun at the kN abbreviation; I agree that no one is confusing kilonewtons with knots, not unless there's a hurricane putting a torque on a broadcasting tower...

No standard abbreviation exists for nautical miles

We can invent one: kn-h. It's knot-hours, which is technically correct but horrific to look at. It's like the time I came across hp-h (horsepower-hour) to measure gasoline energy. :(

if you take all those colonial unit

In defense of the American national pride, I have to point out that many of these came from the Brits. Though we're guilty of perpetuating them, even after the British have given up on them haha

An inch is 25mm, and a foot an even 1/3rd of a metre while a yard is exactly one metre.

I'm a dual-capable American that can use either SI or US Customary -- it's the occupational hazard of being an engineer lol -- but I went into a cold sweat thinking about all the awful things that would happen with a 25 mm inch, and even worse things with 3 ft to the meter. Like, that's not even a multiple of 2, 5, or 10! At least let it be 40 inches to the meter. /s

There's also other SI-adjacent strangeness such as the hectare

I like to explain to other Americans that metric is easy, using the hectare as an example. What's a hectare? It's about 2.47 acre. Or more relatable, it's the average size of a Walmart supercenter, at about 107,000 sq ft.

1 hectare == 1 Walmart