Customize the page limit for JSON API responses for specific paths.
To use, define a set of paths and values as a service parameter in a custom services.yml file. See the services.yml file in this module for details.
Then make a JSONAPI request to your specified path with page['limit']=250 or whatever higher limit you want.
Note that this module is not tested alongside actual pagination of these high-limit requests. The initial use case is needing all items in one request.