The client has not had to pay a cent for any sort of migration work.
Of course, it also means you have to be cautious about problems that dependencies promise to solve (e.g. XSS), but at the same time, bringing in a bunch of third-party code isn't a substitute for fully understanding your own system.
I have worked at employer, where one could have done the frontend easily in a traditional server side templating language since most of the pages where static information anyway and very little interactive. But instead of doing that and have 1 person do that, making an easily accessible and standard-conforming frontend, they decided to go with nextjs and required 3 people fulltime to maintain this, including all the usual churn and burn of updating dependencies and changing the "router" and stuff. Porting a menu from one instance of the frontend to another frontend took 3 weeks. Fixing a menu display bug after I reported it took 2 or 3 months.
From human society's PoV, you sound like a 10X engineer and wonderful person.
But from the C-suite's PoV ...yeah. You might want to keep quite about this.