@gblenn I think it is perfectly valid to have both syslog daemons running and sounds like syslog-ng is there purely to handle shipping the larger suricata logs. With rsyslog coping fine with everything else. TCP isn't necessary either, but it is more reliable if log messages exceed a single UDP payload. So, at a guess, I'd say truncated logs aren't your problem.
However try turning on more suricata log options and see if you break things - see if suricata dashboard still presents recent data as expected in Kibana. Then you can be sure truncating is not an issue.
IIRC with suricata logs being JSON, truncated logs pretty much breaks the entire logstash parsing of suricata. I am not running it right now so cannot check.
EDIT: Also, again IIRC, there are remote syslog options within the suricata package itself. But I cannot remember how or if these should be enabled when you are also running syslog-ng to ship suricata logs. I used suricata for while, mainly as an exercise, but could not justify the increased resources needed with the move to v6.