Hello Jet_Jaguar,
> Is the minor Version relevant in this context, e.g. is 18.1R3-S9 the same as 18.1R1 in regards to upgrade compatibility?
Only the major version is relevant, e.g. 18.1, 18.2, 18.3. You can ignore any R or S parts behind these releases in terms of upgrade considerations.
> For example, can I upgrade from 17.3RX to 18.1RY or 18.2RZ (X,Y and Z being random minor Versions) ?
Since Junos 15.1, all Junos releases are considered as EEOL. Officially you can jump between two releases, e.g. a direct upgrade from 17.3 to 18.1 is supported. However, a direct upgrade from 17.3 to 18.2 is NOT supported, as these would be three releases.
Please note that there is a difference between "officially supported" and "working". It could even happen that a direct upgrade from 17.3 to 20.1 works fine without any issues. This always depends on the used platform and the features you are using.
In general, even if you are upgrading the official way, it is highly recommended to check in your lab if the upgrade works fine, as software issues can always happen.