Can I pin myself if I’m on PCs leave? Or should I wait to arrive at my new unit?

I’d advise you wait until you arrive at your unit. In the mean time, let your sponsor know you made the cut off and to inform your COC. If you a copy of your promotion orders that would be helpful and you should send a copy to your sponsor or gaining unit S-1. I would definitely advise against pinning yourself. The transition from SPC/CPL to SGT usually comes with an NCO induction ceremony. You really don’t want to start off on the wrong foot with your new unit CSM. Good luck and congratulations.

The orders are what matters. The ceremony is just a tradition. Being on the list doesn't mean you are promoted yet as well. If you have promotion orders and it is past the date of rank, then throw it on. If you are still just "promotable" then continue to wait for points.
I pinned SFC without a ceremony. Ceremonies are nice and all, but it is the orders that actually promote/award/assume command

In line with MSG (Join to see) and MAJ Samuel Weber, I would advise against doing so without gaining unit permission. Some folks get SUPER touchy about it.

I picked up SFC while I was in school. Technically, I was TDY from Florida to Fort Huachuca. However my assignment in Florida was actually a detachment from a parent company in.... you guessed it, Fort Huachuca. My parent unit pinned me on a Friday evening after the schoolhouse was done with me. The schoolhouse unit was PISSED and the 1SG ripped off my rank when I showed up with it Monday morning. I explained that my ACTUAL unit had pinned me, not just me pinning myself, and not my TDY unit (the schoolhouse). He refused to find that an "acceptable answer" and threatened to make me wear SSG for a week as "punishment."

To be clear - that was completely unprofessional of the 1SG, and there is no way he could have enforced his threat. But it didn't stop him from blowing a gasket. Even though I didn't pin myself, that 1SG *still* felt like I was trying to "cheat him out of my promotion" (his words). Not sure how he was entitled to *my* promotion, but that is how he felt.

I am not saying that reaction is routine or to be expected - but I am not saying he is the only 1SG who ever had that thought process, either. Better to be safe. If you want family to be there, participate, or even do it themselves, most of the time the unit - gaining or losing - will be happy to accommodate. Just don't leave them in the dark.

You can't wear it until your line number comes up.

If you don't have orders in hand and iPermed, then do not pin. If you do have orders iPermed and in hand, contact your new unit to see if they'll want to pin you or if you can walk in wearing. Some units are very big into the tradition if pinning in front of the unit.

The Army may do things differently but in the Marine Corps you are promoted by your Command. Plenty of stories are told of the guy/gal who promoted themselves on leave and showed up with rank they had not been formally promoted to yet.

I pinned myself years ago on leave. I was checking into HAAF as a E-4 and orders were there for my promotion. I was taking leave and the admin told me I could do it myself and report in as an E-5 instead of E-4 but I better have my new rank on my uniforms.

The Department of the Army has imposed special trust in the patriotism, fidelity and valor and demonstrated leadership ability....

He is therefore promoted from SPC to SGT with a date of rank of...

If you have a piece of paper that says that, with a DOR Before your report date then that is your rank upon reporting

If you made the list while on leave between units you should not show up wearing your new grade.

Congrats!
Now the real work begins.