Doesn’t brooks say that adding manpower to an already late project makes the project later?
On the other hand, you have a few “full stack developers” on hand and you need someone to concentrate on one part of the stack so your current developers can concentrate on other parts.
I’ve found it remarkably easy to hire a front end developer who knows the stack we want to use, who doesn’t know anything about the business besides - here is what we want from the website, give us some mockups and we come up with the APIs you need.