.. a good structured log viewer, that lets me fold/unfold log messages and displays JSON (or Dumper output or whatever) in a structured view.

I suppose you could fold/unfold lines like hiding DEBUG level messages but display everything else, but only keep the full file around for a while. After a day or two, perhaps filter the log file to delete the DEBUG information, and keep the reduced files around a little longer.

Every situation is different, though, and it depends on how much space you have for log files, what detail level you want to keep, how far back you want logs for, and whether there's PII in the log files.