This API has a hard limit of being able to return a maximum of 100,000 users. There is currently no way to page through additional users if you have more users in your org than that. You can request new features and share your use case for having 100,000+ users at https://genesyscloud.ideas.aha.io/.
SCIM is a specification with a specific purpose, which is to synchronize users between systems; it is not an alternative to GET /api/v2/users.
The only available options are the ones documented, which does not include the ability to filter: GET /api/v2/users.
There are search endpoints: POST /api/v2/users/search. It's not directly equivalent to the user endpoint, but you can get back the IDs and look up the users individually.