Anyone here a rep payee for someone on social security? I think I’m going to have to become one for mom. What do I need to know?

Keep receipts and very good records. SS can ask at any time how money is being spent

Is there any reason at all to get ANY mail to her at her facility? Can you not make a general change of address, then deliver mail to her that is personal cards and such? If you are her POA you can do a change of address form at the postal services.
As far as becoming payee, I was POA for my brother, handling all financial matters large and small, and was the Trustee of his Trust as well. I never had to become POA. However, it was made clear to me by SS how to do it when I visited their office. I suggest, now that they are open, that you do just that.
At minimum you will need a diagnosis of incompetency at least, and your papers to say that you are currently her POA or Guardian or Conservator. Thereafter, as well as the meticulous paperwork you already keep acting for your Mom, you will need to make reports to Social Security about her SS check.
So very much depends upon what you are attempting to do for your Mom. Few of us here have gone through the Rep payee process. The Social Security online web pages also make clear the process.

Can't you just setup an online "My Social Security" account to change it?

Yes, I know this thread is two months old. I don't see though where anyone made this suggestion.

As my dads guardian/conservator, i became rep payee when I needed to change the direct deposit from his personal checking account to the conservatorship account. My local ss office was great. Easy to work with. It was during covid when they weren't doing live appointments so we did a phone call. I have always found them very helpful.

Daughter, I'm in a similar boat. The only difference is that I'm a legal guardian, not that that makes it any easier for me. Here's the problem with the "go down to the local SS office" advice (you and I know this, but others who've done this in the past might not): The SSA has not been doing in-person appointments since March of 2020.

I too will probably (reluctantly) have to apply to become rep. payee, and I myself am curious about navigating this process during COVID times. I've read that some offices ask you to send drivers licenses and other sensitive documents over the mail. That's ... not very workable.

If you haven't seen them, there are a few older threads in this forum on the subject, if you do a search for "payee." Be warned that you have to wade through some bad advice, like "Just get a joint account!" (you can't) and "Just have the checks go into your own checking!" (you can't). And few, if any, are from the past year. But I learned a few things from those threads, anyway.

Finally, I totally commiserate with avoiding it because you're scared it won't go well. So many things that I thought would be straightforward matters have been anything but. Good luck to you!

POA worked for me with social security when I had to become payee for my father. You have to make an in-person appointment with them and show your POA papers and if you're planning on changing the account your mom's SS gets deposited in, then you'll have to bring the new account numbers.
To make an address change, go to the post office in the town she lives in. Show them your POA and fill out the papers to change her mailing address to yours. Don't tell the post office she's in a nursing home. Don't tell them down at Social Security either when you go for the appointment about becoming payee. Telling them will just complicate matters. I had no trouble whatsoever. You should be fine. In fact, Social Security didn't even ask me for my father's mailing address because his checks were direct deposit. I think you should be fine just making an appointment and showing your POA papers.

I never contacted SS the whole time I was caring for my Mom. The checks were direct deposited and I was on the acct and POA. So I had no problem paying bills. I did have her address changed to mine but not sure if it was before or after her death. Sge still had a house. I don't see why you can't go on line to the Post Office website and put in for an address change. I would not worry about SS. When my Mom ran out of money and went on Medicaid in a NH, I allowed the NH to become her payee for SS and her pension.

What if all you want them to do is to stop sending payments to the old facility and start sending checks to the home address they have long had on file for her? It seems like it should be a simple request, and I shouldn't need to be representative payee* to accomplish this. Of course, nothing is ever simple in this business.

*There are reasons, having to do with the very restrictive way my guardianship was set up by the court, that I'd rather avoid becoming rep. payee. Just trust me on this, for now, please.

Essentially, I just want SS to return payee status to my mother, and mail her checks to her long-time address. Without being a rep. payee. Will I have a hard time?

I have done it before. Changing to rep payee was an easy process for me. Strongly recommend really good records and receipts

Long story short, it is a horrible procedure. I went to the SS office to change my mother's address and they told me I had to bring her in with me. She was immobile and would have needed to be brought in in a wheelchair. After many calls to SS, they finally agreed to have me put her on the phone to verify her identity. Ridiculous, as it could have been any elderly lady.

I have kept her bank open to accept her SS checks. I have heard horror stories about bank account routing numbers changing, checks no longer going into bank accounts and sons/daughters pleading with SS just to change the routing number. I continue to keep my fingers crossed. They are impenetrable, like Fort Knox.

I do wish you the best of luck, though, and hope you are successful in your endeavors.

Keep money separate, precise accounts and keep receipts.