If you bought a Manila mango and used it for rootstock, that would kind of defeat the whole purpose because you said it is a good rootstock, but how is it going to be a good rootstock if it isn’t on its own roots? Manila mango is more well suited for the bay but that is for its cold tolerance, not for its use as a rootstock because a Manila mango would have to be the original mango tree or a grafted or air layered. I suppose air layered would work also, but the roots tend to be a bit weaker
Try growing a Manila mango from seed instead of a grafted Manila mango and hopefully the genes carry on and then you can graft onto it.
Hopefully this helps a bit