Basically I am REALLY not into being a beta-tester for Microsoft.
Semi-Annual Channel is considered the most stable release channel.
Semi-Annual Channel (Targeted) is considered to be stable but is targeted to known compatible configurations. In other words
Semi-Annual Channel is released to everyone, and
Semi-Annual Channel (Targeted) is only pushed to systems that are known to be compatible with it.
I am unclear on when the major OS update will occur to me and what will happen when it does (never had one happen yet).
Your system would be updated to the current
Semi-Annual Channel build which is currently, as of October 2018, is Windows 10 Version 1803.
As it is now October 2018 - does it mean I will be upgraded to the new OS version any time now whether I want it or not?
Eventually, you would be forced to install the next build, contained within the
Semi-Annual Channel branch. You have Windows 10 configured to wait 365 days, until the next feature update, within the
Semi-Annual Channel branch will be installed. When the update will be installed exactly on your system cannot be determined with the information you have provided. You really should be running 1803 at this point anyways.
All releases of Windows 10 have 18 months of servicing for all
editions--these updates provide security and feature updates for the
release. Customers running Enterprise and Education editions have an
additional 12 months of servicing for specific Windows 10 releases,
for a total of 30 months from initial release.
Source: Semi-Annual Channel
What version will I be upgraded to? 1803 or 1809?
You would be upgraded to which version is in the
Semi-Annual Channel branch at the time of the upgrade.
How does upgrade really happen?
The feature update would be downloaded in the background, and since you are running 1709, the majority of the installation would happen in the background. At some point, you would be prompted to perform a restart to finish the installation of the feature update.
Will I be given an opportunity to make a full OS backup before?
You make a habit of making routine system backups.
Will Windows just quietly download everything required and start it whenever it feels like?
Yes; Feature updates are required since you are running Windows 10 Professional.