Just say Y'all, it pretty much covers that

I prefer "fellow human" spoken in a slightly suspicious manner.

Greetings, sentient!

I use 'homie' sometimes. It's not always the right option but it's another arrow in the quiver.

Folks. It's my go-to.

Mate

This is hard.

Buddy? Sounds dismissive.

My friend? Sounds like you're a scammer or a slimy salesperson.

Pal? Sounds mildly aggressive.

Dude? Arguably masculine.

Gen Alpha might have it right -- "bro" seems to be gender neutral and used by both boys and girls.

Marcie calls Peppermint Patty Sir, so maybe that works. Only half joking.

The scousers (people from Liverpool, England, UK) have a very useful one in their dialect, "Youse" (pronounced Ewes, like the sheep)

And it's gender neutral, but also double as a group pronoun

Youse gonna use that?

(Person, are you going to use that?)

Hey youse, heading out?

(Depends on context, can be singular, can be group, either "Person, are you going to go outside?" or "People, are you going to go outside?")

It's v useful

Lean into the southern-ness and call them Honey, Sugar, or Sweetie?

It gets easier to pull off the older I get.

I've written about this before, but "neighbor" or "cousin" or "my friend" usually work. If you can have a bit if fun with it, "Senator" is great. If plural, "folks".

I like referring to all as boss or chief.

My partner and I landed on "Captain" (or the truncated "Cap'n"). But that might be all the star trek we watch bleeding over (I never thought having "make it so, number 1" whispered in my ear would be so hot).

I like to use “dude” or “guy” in a neutral way.

Sa'am.

I can't think of one that would be as universal as the sir/ma'am pair.

I think it's probably most polite to ask them.

Or at least to assume one or the other, use it with an enquiring tone, apologise if corrected and move on.

S'mr'amr

Just invent something. English isn't even a strictly gendered language. Just don't expect anyone to want to use a lame word like xir or some other derivative hipster shit you gotta stick the landing. You can just choose to create new words and see if something catches on noone will stop you from trying. People do tend to be masc or fem for a reason and probably don't want to be treated like something inbetween so keep that in mind.

I was told "dude" counts as gender neutral these days, but people seem to differ in opinion on that one.