Retain your Elastic ingest pipeline, including Logstash. Simply fork a copy of your data to S3 for cheap and long term storage and Sneller will access the data from there.
Use Sneller's Kibana proxy to transparently translate Kibana and Elastic DSL queries into SQL for the Sneller cluster. There's no need to translate or rewrite your dashboards and queries.
You can now consider migrating to other dashboards (e.g. Grafana, Superset, Tableau, Looker) that are SQL compatible, via a JDBC driver.