there are other ways to share code than a monorepo, namely independently versioned packages. you say dependency hell but if you're in control of everything you're in the best position to avoid that if you put thought into it. with a monorepo you could actually get worse results in some cases. say i change something in a graphics api for sudoku and suddenly the zoom clone doesn't work. yuck. i only buy the monorepo argument for systems of code that actually need to be deployed in sync, like server/client stuff or larger. iirc google has a massive monorepo for nearly all of their production code because basically all of it works together (server/client stuff, auth, etc)
also a monorepo like this is worse for human interaction, if you care. what if someone only wants to pull and work on sudoku? now they have to get everything else. issues and pull requests are also more annoying to varying degrees depending on use.