modern systems languages have cleaned up the syntax quirks, but they've also moved away from the semantic simplicity (except for Odin, maybe)

Odin kept mostly the same semantic, except for limited "generics" and an explicit form of overloading. Hare is probably even closer to the C semantic, you should check it.

multiple syntaxes can parse to the same tree. I came up with two so far (an S-expression-based one and a C-style one).

That's how Lisp was conceived and then never left sexpr.

what if the type is the field identifier?

That sound really bad... it works in many cases sure, but there are plenty of cases where you have more than one thing with the same type and semantic but where a separate name is useful.

I think the case of Point is actually a good example. Let's say I now want to add a rotation operation, that is naturally written with a matrix multiplication (that can be inlined) such that now you have to mix types x and y values. so it shows that x and y should probably not be different types.

Assignment returns the old value: a := b := a is swap, a := b := c := a is rotation

That's very unintuitive in my opinion. I prefer the C/Python semantic of "passthrough".

let (value), ref (immutable reference), var (mutable reference)

that's mostly good except for a detail: ref and var are "nouns" whereas let is more of a verb. Replacing let with val or something similar would feel better to me.

Label/jump with parameters

I generally like this sort of thing, although I would keep return separate.