Hi @arblr,
I can see two possible problems by now.
First, the versions. You mention that you are using Journalbeat 7.5.0 with Logstash and Elasticsearch 6.3.0. This might work, but it is not supported, you can find here the comatibility matrix: https://www.elastic.co/support/matrix#matrix_compatibility
You would need to use Journalbeat 6.x, or upgrade your Stack.
Second, it seems that you are using both Journalbeat and a journald input for Logstash. Why are you doing that? In principle you would only need to collect logs from journald using Journalbeat (or the Logstash input, but not both). The trace seems to indicate that the Journald plugin for Logstash is not being able to open the journal file.
In the logstash configuration I cannot see any beats input configured. If you want Beats to send events through logstash, a beats input should be configured there.
Also take into account that if you are not doing any additional processing in Logstash this piece is not needed and you can directly configure Elasticsearch as an output in journalbeat.