"invasiveness" is a relative issue. When we've adopted Volto, we were actually using npm and a script that solves the workspace problem. I think it may be possible, with some small effort, to compile and use a Volto project using just npm (but only for production).
Now, yarn in itself is nothing special. Yarn 2, npm 7, pnpm all have support for monorepos and would work fine for us. There may be (and I think I tackled those issues in the above PR) some code that relies on the specific way yarn does the "hoisting", which is "lifting up" all the dependencies up to the top level of node_modules.
I'm not a fan of the hoisting, at all. It causes all sort of issues, phantom dependencies, etc. Big fan of pnpm's algorithm and method of installing.