Sorry to be late to the party, but you may be interested by json-rql, which is very simply a serialisation of SPARQL, but intended to be used in a GraphQL-like way. In particular, being JSON it’s easy to construct in javascript, and easy to validate on the server, for example using JSON Schemas.
json-rql is just a language and does not have the framework and caching support that you want, though those things should be fairly straightforward to put together. The npm Javascript library offers a reference translation to/from SPARQL, and there is a Java AST library including integration with Jena.
My project m-ld uses json-rql for its API, with the intention of hiding complexity for common use-cases. m-ld is decentralised so it doesn’t have a server-side, but it’s easy to envisage. Happy to talk about it more if it’s of interest.