Somewhat off topic - keeping older Macs running

[bunk3m]

Hi all,

Now and again, I read of people on the list keeping older Macs running 
using Macports.

I have a perfectly functioning iMac which can't update past 10.13.6, yet 
there are nearly monthly security issues.

I have been running Ubuntu on an older Macbook Pro with a C2D processor 
but the change to Ubuntu Linux might be too much of a challenge for my 
wife on the iMac.

Is there a discussion forum or website that people recommend where there 
are suggestions on how to keep older Mac hardware running and secure? 
I've done some Googling but I can't seem to find anything.  (It is 
probably an issue of not knowing the right search terms)

Or are there recommendations on how to use Macports with these older 
devices to keep them operating securely?  I'm always concerned that the 
older unsecure software in the /System and /Library folders are used as 
default instead of the updated Macport versions.

Thanks in advance for any suggestions.

B.
PS I'm on digest mode.