--max-old-space-size=8192 should suffice, we run Kibana 4Gb in dev mode (yes, it's a lot and we are working to reduce this number).
Do you have any custom plugins installed? Could you try to disable them in kibana.yml file with {pluginName}.enabled: false? Could you also disable some unnecessary Kibana plugins?
https://www.elastic.co/guide/en/kibana/current/disable-plugin.html