OK for now - sort of...what a confusing subject !!
After I responded to one of their marketing email addresses & added 'support' to it, they replied to my query within hours - amazing & maybe that is because their web contact form is inoperative or ignored ??
Here is what they told me:
Thank you for your email. Ubuntu 18.04 LTS is still in its standard
support till April 2023. After that, 18.04 will go into the Extended
Security Maintenance for another 5 years till April 2028.
So therefore I now conclude...
Big whoopty-doo.
There is absolutely zero hurry or any necessity a'tall to leave 18.04.5 behind for quite a while yet;
Therefore my trusty & great running 'daily driver' PC can stay as-is.
I'll set up a replacement system for taking its place - but now it'll simply be done at my leisure along with care & absolutely zero urgency to have it all done.
What the heck - April 2023 is quite a ways off & being as old as I am & what with the craziness of the world at large these days ANYTHING could happen 'tween now & then !!!