iMac restarting unexpectedly

The OWC SSD is a 3G model which indicates it only supports up to SATA II speeds which is only about 250MB/s max transfer rates. So the speed reported by EtreCheck is Ok.

The DriveDx report for the OWC SSD does show that the SSD currently has three Pending Blocks awaiting reallocation. It is possible if a macOS system file is located within those three bad blocks, that it could cause a problem. There has been a single "Offline Uncorrectable Block Count" which means at some time in the past there was an uncorrectable error. This may have occurred with the previous bad block that has already been reallocated which can sometimes happen if the bad block is not reallocated quickly enough if the block failure is severe enough. I doubt it is due to the current bad blocks awaiting reallocation since there is only a single uncorrectable error. I don't know when the SSD will trigger the reallocation of those bad blocks. In theory the SSD's controller should do this quickly (or at least when the SSD is not being actively used), but if it behaves like a hard drive, then the bad blocks won't be reallocated until data is written to those locations. I honestly don't know how/when this OWC SSD performs this operation.

There is also a single UDMA CRC Error which usually indicates an issue with the cable, but a single error is nothing to worry about and may have occurred with the SSD connected externally or to another system (perhaps even during manufacturer testing). There is no way to know without a former baseline report. This report will become the baseline for future checks.

FYI, it is perfectly normal for an SSD to encounter some bad blocks during its lifetime so this is not really a concern unless a lot of blocks go bad at the same time.

You may want to consider enabling TRIM on the SSD. While I know older versions of the OWC SSDs did not require TRIM, you are using a newer model of OWC SSD which utilizes a different controller, so it is hard to say whether they are the same. You can enable TRIM by using this Terminal command:

sudo  trimforce  enable

This will prompt you for your admin password. Nothing will appear on the screen while entering the password. Press the "Return" key to submit the password. You may also need to run Disk Utility First Aid on the "Macintosh HD" volume after the reboot so a TRIM command is performed immediately, otherwise it may take a while before macOS performs a TRIM operation.

Your external Hitachi hard drive has been running hot for a while (21K minutes), unfortunately we cannot tell when this occurred from the report. Highest temp was 67C while the highest average is 65C. The drives recommended max temp is 60C while the Max Limit is 70C. While the rest of the Hitachi's health attributes look Ok, there are some references to some errors outside of the attribute table such as some read recovery attempts, and a couple uncorrectable errors, as well as two "IDNF" error at 6,278 run time hours (mostly irrelevant now since the drive now has 17K hours -- drive may have been bumped)). I'm not certain how to interpret those couple items since I don't usually see reports with those errors (I usually only look at the main attribute table).

Except for the excessive heat I'm not sure there is any concern here. You may be able to retrieve a temperature history of the drive by using the Terminal command line to collect a temp history report using the core command line utility within the DriveDx app. Within the Terminal app begin by typing "sudo " (leave at least one space after the "o"), then drag & drop the DriveDx app onto the open Terminal window to auto-fill the correct path. Press the "Delete" key to remove the space at the end of the line, then continue typing the following:

/Contents/Resources/smartctl  -l  scttemp  /dev/diskX

You will need to replace "diskX" with the correct drive identifier for your Hitachi drive which you can retrieve using Disk Utility (or maybe even DriveDx). This drive identifier can change each time you re-connect the drive so you need to check immediately before issuing the command. In the DriveDx report the drive identifier was listed as "disk2". The final command will look something like this:

sudo  /Users/hwtech/Downloads/DriveDx.app/Contents/Resources/smartctl  -l  scttemp  /dev/diskX

The Kernel Panics were most likely caused by the Avast anti-virus software you had installed. Some of those Kernel Panics referenced memory that had been modified after release. That either means some third party software did this (anti-virus most likely), or some app has a bug, or you have bad memory.