My grandmother almost always tells me daily that she wants God to take her. Is it bad that I am at my wits end?

The truth is that your Grandmother may now be ready to die, would PREFER to die. You are too young, perhaps, to understand that this could ever happen but as a nurse and as a 78 year old woman I can assure you that it DOES happen. I remember my father telling me that life was now too hard, that he only wanted peace and rest, that my mother's trying to get him up, on the scale, make him things to eat was just more exhaustion for him; he tried for her, but he was done. He had had a wonderful life; been a good man, was ready for peace.
More recently, before his death in May, I heard the same from my brother. It wasn't a complaint. It was that he was ready, didn't wish to go on down the long slow slide with loss after loss, and finally the loss of all he is, his memory.
As a nurse I heard this over and over again.
I agree that this is not a discussion for young children, but it should be one for you now to consider. Your grandmother needs her opinions and feeling valued, not turned around with "you hurt my feelings". These are her honest feelings and wishes. At the least she needs to hear from you "I am so sorry you feel that way; I so wish I could change things for you; we just aren't given an exit button like on the subway".
As a nurse I heard all the things that my patients felt they could not say to their families. This was one of them. Your grandmother has chosen to say it. She is miserable in a way you cannot yet understand; there is no upside to aging.
Talk with her, allow her to say her own truth, tell her you are sorry she feels this; ask her if there is anything she can think of that may help that you can do for her or with her.
I am sorry. We tend to be as a species a "fix it crew". When we cannot fix it we get frustrated enough that we want to deny it is a fact. But your grandma's feelings? They are a fact.
You also are recovering. Allow yourself the time to stay home and do that. Grandmother will have to access the resources available to her while you heal. If her negativity is more than you can bear often to be around, then do not be around it often, and let her know gently why.
As far as taking her into your home I think you know, with the frustration you already feel, that this is not a good idea for you or for her.

Your grandmother is feeling depressed and useless and unwanted in this world. She may not be able to do much anymore so sees no point in being alive. If she is still living independently in her own home, she must be physically able to take care of herself. If not, she'll have to hire help.

When she says she wishes God would take her, you can tell her God will take her when he is ready for her. He must not be ready yet.

My dad often said the same. At first it was hard to hear but over time I came to understand that at his age and with his chronic health issues he really was tired of this world and wanted out. We may all reach this point one day and just haven’t yet lived long enough to realize it’s coming for us. So when he said it I’d say “I understand” or “I don’t blame you for feeling that way” and go on to talk with him about another topic. Though it may make you sad, death is real and there’s nothing wrong with acknowledging your grandmother's feelings on it. That’s not to say going on and on about it. My dad did also take Zoloft his last few years, it was a help with his sadness over life. The doctor presented it in a way that didn’t call it depression to help dad accept it. Many people of this age don’t like the concept of depression. Don’t come running constantly for your grandmother, she needs to see her need for other help. I wish you both calm and peace

I am sorry that you are struggling with this situation.

I commend you for wanting to protect your son.

You work very hard and deserve time for yourself and your son.

I understand that she has issues in her life. I understand that you love your grandmother.

Your feelings are completely normal. Your grandmother’s behavior is draining all of your energy.

How old is she? What are her health issues?

She sounds very depressed. Would she be open to speaking to a professional therapist? What about taking medication for her depression?

Limit your time on the phone with her. It’s fine that you became frustrated. It’s normal.

Sounds like your Grandmother may just want a sympathetic ear, some emotional support.

But also sounds like your tank is on empty... nothing to give. Understandable. Some self-care is needed too!

Both having survived Covid is a scary & massive thing! Be kind to yourselves ❤️❤️.

Regarding the dying talk;

It really depends on the person, their personality & the mood but when my Gran talked like that, I would hold her hand & nod. I would say "God will take you God is ready". Sometimes it's like people are asking permission. Sometimes it is depression, or can be exhaustion or maybe fear of the future. Ask her why she feels that way.

You don't need to solve all her problems. Sometimes a kind visit is enough.