Retrofitting an API is a context specific problem, with few things that generalize but the above aren't the hard parts, just the complex ones.
I am going to point you to a Public Policy book here, look at page 32 and their concept of 'wicked problems', which applies to tech but doesn't suffer the problems with the consulting industry co-opting as much as in the tech world.
https://library.oapen.org/bitstream/id/f3358d25-56c0-47f2-90...
Obviously most companies won't accept the Amazon style API edicts that require Externalizable interfaces and force a product mindset and an outside in view.
Obviously an API is still a "Cognitively complex problem" as defined by the above.
But what makes most API projects fail is the fact that aspects arise that result in ‘wickedness’ A.K.A intractability.
API gateways, WAFs, RESTful libs with exponential backoff with jitter....etc... all exist and work well and almost any system that you have a single DB you are concerned about killing is possible to just add a anti-corruption layer if you don't have too much code debt, complicated centralized orchestration etc....
But these general, but difficult tasks like adding an API almost never fail due to tech reasons, but due to politics, poor communication, focusing on tech and not users, unrealistic timelines, and turf battles.
You are correct that "APIs require thorough documentation or they're useless" but more importantly they need enforceable standards on contract ownership, communication.
The direction of control, stability, audience, and a dozen other factors effect the tradeoffs there as to what is appropriate.
Typically that control is dictated purely by politics and not focused on outcomes.
That is often what pushes these complex problems into failed initiatives, and obviously this is far more complicated. Often orgs can't deal with the very real uncertainty in these efforts and waste lots of project time on high effort, low value tasks like producing gantt charts that are so beyond the planning horizon that they can only result in bad outcomes at best.
The point being is that unless you are on the frontier of knowledge and technical capabilities, it is almost never the tech that causes these efforts to fail.