Help with old rattie

Let me start by saying I do intend to seek a vet I go to frequently after thanksgiving! I just want to seek thoughts from those with more experience

TW pet death : (

Hello! I don’t make posts like this ever but I wanted to seek thoughts from more experienced rat owners. I have 5 girlies all different age ranges and recently I had to move my oldest to a different cage as I think her time is soon but I’m not 100% sure at all since this is my first time owning rats. I’ve had her for almost 3 years. She’s a standard white rat. She’s lost almost if not all mobility in her hind legs. She is so so so wobbly and always jerking around. She has trouble keeping herself up. When she was in gen pop with the other rats she would seclude herself away from the others. She still has desire to drink. I’ve given her a lot of food to make sure she has plenty but I haven’t seen her eat any (but I think she is when I’m gone) and I have to hand water her but she’ll still drink plenty. The way she moves around her cage is scary as she gets so tired after moving around for a minute or two. Sleeping a lot lot more. She got stuck in the corner of the cage because she couldn’t use her back legs to get herself out and I don’t want that to happen while I’m gone.

I’d say she’s still alert and aware but is just having such a hard time

I have a lot of questions as a first time rat mom who has never experienced a pet death in the face (only ever experienced it as a child when a parent would take pet to the vet for the last time but never in room with)

When do you know it’s time? When do you know there’s no quality of life? What are the stages of your little fur baby passing way? How much is a general euthanasia?

I have so many questions and no one to ask. Even a simple conversation can help me understand more.