Hi @Cazzza
There is a lot of good evidence that pets can help those with MI; I believe most of these studies involve family pets, not trained service dogs.
My husband loves our pet dog dearly and walks her regularly. On the internet you will find a number of recommendations regarding best breeds for this, a number are lower maintenance than others, some less prone to health problems.
I don't know enough about service dogs except they are really expensive and hard to acquire.
There are a number of people on the lived experience side who have much loved pet dogs. If I recall there is someone who may have posted in the carer "Dogs rock" social thread who does have a service dog.
Having a clear understanding as to who is responsible for walking, feeding, vet bills and poop pick up would probably be best sorted before a pet moves in. Whilst Mr D walks our dog, I mainly take care of the rest and when he is acute have had to take her walking too.
https://www.npr.org/sections/health-shots/2016/12/09/504971146/pets-help-people-manage-life-with-serious-mental-illness