Please post several of the Kernel Panic logs if they are available. The file names will begin with "kernel" and are located in "/Library/Logs/DiagnosticReports". Post them here using the "Additional Text" icon which looks like a piece of paper.
My bet is the SSD is failing since the SSDs on this particular model have a much higher rate of failure than other models, and the symptoms fit perfectly since the issue is most likely to occur either when powering on the laptop after it has been powered off for a while, or when waking the laptop from sleep or hibernation. The SSD typically takes too long to go ready under these conditions. If I'm correct, these are the first & only symptoms/warnings, and at some point the SSD will fail completely (could happen at any time). At one time Apple had a free SSD repair program for a very small number of this specific model laptop, but I believe the program has now expired. I have personally seen laptops of the exact same model which did not qualify for the free repair program have these SSD failures. The Kernel Panic report can confirm this specific SSD problem, or they may be able to point us to another possibility.
Make sure to have a good backup now while you still can.
Of course like @Grant mentions, there are a lot of other possibilities as well.
You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.