Upgrading every 18 months is definitely too seldom. For roughly the last
6 months, you will either need to backport security fixes yourself,
which will at times be highly non-trivial and in any case a lot of
completely useless work, or you will run unpatched.
In my experience, upgrading every 6 months is better than upgrading
every 12 months. It is not more work to do: You need to do both upgrades
one after the other anyway, and you need to adapt the system configuration
after each step anyway. Half of the time, you will have an overall better
system. The danger to forget details about how to efficiently do the
upgrade is smaller if you do it twice a year.
Huh? You must be joking.
The part about "simplicity" is a nice laugh. :-)
The part about "open source" is rather a matter for sadness, though.