> The recommended advice is to start with semantics first. Syntax will change, there is not much point fixing it down too early.

It's actually the reverse, in my opinion. Semantics can change much more easily than syntax. You can see this in that small changes in syntax can cause massive changes in a recursive-descent parser while the semantics can change from pass-by-reference to pass-by-value and make it barely budge.

There is a reason practically every modern language has adopted syntax sigils like (choosing Zig):

    pub fn is_list(arg: arg_t, len: ui_t) bool {

This allows the identification of the various parts and types without referencing or compiling the universe. That's super important and something that must be baked in the syntax at the start or there is nothing you can do about it.