TBH, I am not a fan of frontend/backend split either: ideally, you'd still be splitting per component/service, so frontend and backend for a single thing could live in the same place: you get the benefit of seeing the API in use with each PR, without the costs of monorepo otherwise.
Most projects start out as monoliths (which is good) and splitting up on this axis is unfortunately very hard/costly.
This is why I'm a fan of tools like Bazel, where you can still get most of the tooling benefits from a single repo, but get testing speed benefits (and, if you roll that way, the design benefits from the separation) of a multirepo setup.
Unfortunately it's hard for me to recommend Bazel, it's such an uphill climb to get things working within that system.
Most projects start out as monoliths (which is good) and splitting up on this axis is unfortunately very hard/costly.