Problems with external SSD on iMac late 2014

Since my internal SSD drive died, I acquired an external M.2 drive with an enclosure and connected it to one of the USB 3 ports in my iMac. I made the drive bootable and restored from Time Machine. The internal HDD is also a boot drive.

The problems are:

• Sporadic crashes, with kernel panics, sometimes no problems for days, sometimes several times a day
• The only way of booting from the external drive is by holding the ALT key, even if the system reports that drive as the Boot Drive. Also, regardless of whether I press the key or not, the system takes around a minute just waiting till it decides to boot. Every time the SSD external drive shows up as the "default" boot in the drive picker interface.
• Whenever I restart the computer, it will always go for the HDD, regardless of if I hold the ALT key. The only way of having it boot from the external SSD is by shutting down.
• When the system goes through one of the panics, I can do two things: force shutdown by holding the power button or go into HDD world and shutdown from there.
• When I run bless -getboot from a terminal I do get the SSD back if I'm on the SSD World. In the HDD world sometimes I get it right (SSD) but sometimes I get an error of device not present or something like that but if I unplug and replug the drive it will show it as the boot device if I run it again
• This is the output from bless -getboot --verbose (now having boot off the external SSD):

EFI found at IODeviceTree:/efi

Current EFI boot device string is: '<array><dict><key>IOMatch</key><dict><key>IOProviderClass</key><string>IOMedia</string><key>IOPropertyMatch</key><dict><key>UUID</key><string>E04A2FD9-A2CC-4171-8FB3-02D039A0C0C9</string></dict></dict><key>BLLastBSDName</key><string>disk4s3</string></dict><dict><key>IOEFIDevicePathType</key><string>MediaFilePath</string><key>Path</key><string>\FA2D3468-BB07-4F33-8263-CA72573F0308\System\Library\CoreServices\boot.efi</string></dict></array>'

Boot option is 8BE4DF61-93CA-11D2-AA0D-00E098032B8C:Boot0080

Processing boot option 'Mac OS X'

Boot option matches XML representation

Found device: disk4s3

Disk boot device detected

Detected APFS volume.  Skipping Boot!=Root check.

No auxiliary booter partition required

System partition found

Preferred system partition found: disk0s1

Returning booter information dictionary:

{

    "Auxiliary Partitions" =     (

    );

    "Data Partitions" =     (

        disk4s3

    );

    "Preboot Volumes" =     (

        disk4s3

    );

    "System Partitions" =     (

        disk0s1,

        disk3s1

    );

}

/dev/disk4s3 is a preboot volume

Substituting found system volume /dev/disk4s2

/dev/disk4s2

• The kernel panics always involve something like along the lines of a page fault:
• panic(cpu 0 caller 0xffffff80143c1596): Kernel trap at 0xffffff8015c786b7, type 14=page fault, registers:

• My hypothesis is that for some reason the drive "goes away" and hence the crash
• Lastly if I plug the drive on my MacBookPro none of this happens, boots are really quick and have never got the behaviour. Having said that I don't use it as much as the iMac

What do you think? What can I try to sort it out?

I have thought about putting the drive inside, with an adapter, but it's a complex endeavour.

A different, semi-unrelated question. I had thought about connecting to the TB2 port but it's impossible to find cables (should be USBC to TB2). The apple one won't work as I need "male" connections both sides. I think there is an option to buy a male-male TB2 but that would make the combination ridiculously expensive.

Many thanks!