Is there a way to update my wife's POA or is it too late?

Read the vesting power very carefully and see if it allows you to involve someone else to assist you.   My father's did.

Go to your state's attorney general website and see if they have DPOA and DMCPOA, these are completely legal and binding.

Take it to a UPS store or similar and have her make her mark and get it notarized AND witnessed. They never check anything except ID, because they have to put it in their book.

If your wife is able to parrot enough of the basic understand as to what a POA and you find the right lawyer they will sign off on it. Guardianship is a pain, as our family lawyer told us he rather bend the rules a little and go down the route of POA if the patient is compilate, not difficult, and no signs of abuse are present then put a caregiver through the minutiae that is often unnecessary when it comes to providing care. It just another added expense and burden to a caregiver dealing with enough on their plate.

Speak with a lawyer, it will take time but you will find one that is understanding towards the situation. Another point our family lawyer brought up, many lawyers do find guardianship in most with general family issues pointless, like yours and if the family keeps their mouths shut then many cases no one is none the wiser.

I believe it is too late for your wife to do anything as POA is done ONLY by a competent adult who understands what she is doing. I am surprised a "second" was not suggested by your attorney. Upon your death it will be a simple matter for your son to take these papers to an attorney and to get guardianship. You might want to attend an elder law attorney with your son and your wife if you think your wife can understand what she is doing and an attorney can judge that she does. It isn't the lack of a legible signature that's the problem. It is a matter of level of understanding your wife has. Incompetent adults cannot legally sign legal papers they cannot understand.

It would depend on how far into her ALZ diagnosis she is. Being able to sign a new PoA doc has less to do with memory retention than comprehension. At an elder law attorney appointment she would be interviewed privately so that the lawyer could assess her ability to understand what she'd be doing if she signed a new PoA. She doesn't necessarily have to remember it, mostly understand it. Another option is for you to resign as PoA and your son to pursue guardianship.

If your wife is lucidly able and willing to add a second party, you may be able to add a second party, but if your current POA was drafted by a lawyer and notarized, have a lawyer involved for the potential change, as well.