Our codebase (latest tree) is similar, but switching to git it's the total history size that is the problem. Our history is well over 25GB which git doesn't handle very gracefully.
Yeah, the working tree works well to have locally, and that's what's done with svn currently.
The problem is that I also want a fast log/blame for any file back to the beginning of time - but I'm ok with that requiring devs connecting to the server containing the history (as with svn).
I also haven't found a way to make git work smoothly in shallow mode as the default, e.g can I make checkout of a branch always remember it must be shallow? Can I make log use remote history when necessary etc? I don't want to fight the tool all the time because I'm using a nonstandard approach.
This solves the next scaling problem of avoiding managing the whole working tree. (without requiring narrow clones which have significant downsides)