This is pretty much what we’re building at Dopt (dopt.com). A user adoption platform built for devs and PMs. We’re a visual flow builder that lets you define the logic and targeting of your flows with APIs and SDKs that let you build with your own UI components.

We’ve seen a ton of discontent with the existing tools as you mention (they’re brittle, don’t match the brand, force you to use a limited set of modalities).

We’ve seen a lot of companies forgoing them to go the route of building their onboarding flows themselves, but then they’ll run into another set of issues (logic/targeting/state management is complicated and hard to build/iterate on, non devs struggle to understand what users will actually see).

As you mention it makes it hard to move quickly and test experiences, which is what you need to do if you want to figure out what works for your product and users.