I can highly recommend JSONedit. It serves as both a JSON viewer and JSON editor.
It features text, tree, and list views.
It is portable software and is free of cost (freeware). It even features a dark theme/mode.
The only issue I've had with it is that it crashes for me when I open very large (hundreds of MBs) JSON files. I'm hoping the author has time to fix that issue.
I've never had any problems with it with smaller JSON files.
If it wasn't for the crash with large files, I would give it 5 out of 5 lizards. With the crash when opening large files, I give it a highly respectable 4 out of 5 lizards.
JSON edit is available here: https://tomeko.net/software/JSONedit/
Here is a nice screenshot from the author's page:
After spending some time composing this answer, I re-read your question. I realized that I initially missed a key word, "not" (as in file is not valid JSON), when quickly reading your question the first time. My apologies.
So I took your exact data and tried loading it into a JSONedit. As expected, it could read it and display is nicely, but not process it as JSON.
I then tried tweaking your data a little bit to see if I could quickly massage it into a valid JSON format. I only had a few more minutes of spare time, and I was unable to get it working.
I think you could definitely write a simple pre-processor to process your log files and then send the output to JSONedit, but the exact format you provided is unfortunately not compatible.
If this answer is of any use to you, great. If not, just leave a comment under it, and I'll delete my work.