> As a final observation, you can split big repositories into smaller ones quite easily (in Git anyway) but sticking small repositories together into a bigger one is a lot harder. So start out with a monorepo and only split smaller repositories out when it's clear that it really makes sense.
If you only need to do this once, subtree will do the job, even retaining all your history if you want.
I'm not sure what the easier way to split big repos is.
In practice, I can tell you from first-hand experience that this isn't all that simple in bigger, organically grown cases (you'll have many other things to consider if you want to keep the history in a useful way). Especially the broken branching model of SVN and co. is a problem here: In the wild, it immediately leads to "copy&paste branching" (usually through multiple commits. Migrating that to Git or Hg and splitting it up can be a challenge.
If you only need to do this once, subtree will do the job, even retaining all your history if you want.
I'm not sure what the easier way to split big repos is.