Here below is a sample of a solution through which i managed to install MX KDE onto an SSD connected via usb:

"MX Install" to external drive with enabled UEFI and BIOS booting.

Boot in UEFI from a MX LiveUSB.

Prepare partition layout of the external drive in Gparted:

1st: Device -> Create Partition Table: GPT ( not MSDOS !)

2nd: Create partition: bios_grub (this is needed to smoothly allow BIOS booting on GPT):
Size: 1MB
File system: unformatted (scroll down to last entry),
Partition name: bios_grub
Label: leave empty
Click: Apply all Operations!
Right-Click or Menu->Partition: Manage flags: select "bios_grub" (important!)

3rd Create partition: ESP (EFI System partion"
create: 512 MB, format Fat32
Partion name "espUSB", File system label "espUSB"
Manage flags: select "esp+boot" (important!)

4th Create further partitions as needed :
next partitions:
boot ext4 512MB (only needed for encrypted install,otherwise ignore)
root ext4 32GB or bigger
home ext4 nnGB
label all partitions accordingly
E.g. I used rootUSB, homeUSB

Now run the "MX Installer"

and select the prepared partitions for root, and home on the USB device
On GRUB install select "ESP" and partition "espUSB"
When the Installer has finshed do not reboot yet "un-select" to reboot on the last screen.

Now, to prepare BIOS booting:
Open "MX Boot Repair"
and install Grub loader for BIOS booting
Only select "re-install" GRUB, and select MBR of the USB-device.

- - - - -

Now, KDE Neon is different, and the regular updates are enticing.
Can similar thing be done for KDE Neon all the while assuring the ongoing updates as they come along?

Think the tricky part is that last paragraph and how to get KDE Neon to install GRUB loader for BIOS booting and only select "re-install" GRUB, and select MBR of the USB-device.

Hope someone might have an idea...

Hope someone will drop by sometime in 2021!

Did manage to install KDE to the SSD but would not boot at re-boot.

Just can't figure out which options to choose in KDE boot repair... so as to boot within any computer.