I will get a furry of downvotes for saying it but the cause of your first three problems is git, not the mono repo approach.

Git only allows to check/commit/view to the entire repo at once. Then, some git operations are superlinear with the number of files or revisions, they are slow on large repo to the point of being unusable.

It's mandatory to have operation on a per file or subdirectory level in a mono repo approach. Companies that have mono repos all built tooling to support it. CVS/SVN used to do that out of the box but everyone hate them now.