I'd recommend sticking to POST requests with body that fetches the data you need. I know it's semantically odd because the POST request not creating a "resource

I'd recommend sticking to POST requests with body that fetches the data you need. I know it's semantically odd because the POST request not creating a "resource" on the server. But it's practically very useful as it'll work well with client libraries, load balancers etc. You can get some inspiration from GraphQL for your use case. All GraphQL queries are POST requests with body specifying what data the client needs.