The entire point of the way the original trilogy was developed is that we’re dropping into an existing universe.

Yes. What does this have to do with anything?

Snoke rising to power in the 30 year gap in time, especially since he came from the Unknown Regions, fits in well enough without having to give him a backstory.

How did Snoke did Snoke come to be? Why did he create the First Order? How did he manage to get the resources to create the First Order? How did it get so powerful? How didn’t the New Republic notice the growing extremist organization that wanted to destroy them? How did Snoke contact old Empire generals and make them come to him? Why didn’t the New Republic destroy them the moment they heard of their existence?

It does not fit, in any way.

There’s nothing about him that doesn’t fit, we just don’t see every detail of how he fits.

Nothing about him fits.