Commercial and closed source software is no safer, and may even be using the same foss third-party libs under the hood that you’re trying to avoid. Just because foss licences generally require you to disclose you’re using them, it doesn’t mean that’s what actually happens.
And even if, by some miracle, they have a unique codebase - how secure is that? Even if an attacker can’t reach the source, they can still locate exploits and develop successful attacks against it.
At its core, all software relies upon trust. I don’t know the answer to this, and we’ll be here again soon enough.