I am glad you got it working.
Filebeat modules are made for smoothing the getting started experience of new users. So by default its pipeline does not support custom log formats. However, advanced Filebeat users can modify the patterns in module/nginx/access/ingest/default.json and their own format. After the pipeline is updated, it needs to be loaded to the Ingest node. Right now in order to update an existing pipeline, you need to delete it manually and then load the new version. But in the next release filebeat.update_pipelines is introduced. If it's set to true pipelines are always updated.