Git and multi-repo were a mistake. Git’s packfile is a disaster.
Checkout Sapling and ReviewStack. They look promising.
What makes Sapling SCM different & an improvement over git; from a little googling it seems to be a skin over the top?
They’re a decent bandaid for a company already suffering from multiple-reposis. Cross-repo changes, stacked PRs. It’s not the ideal starting point but it’s less pain than the status quo.