Slight misunderstanding here; AFAIK starting with LEAP15 openSUSE should have a minor release about every 12 months in sync with SLED/SLES, but the previous point release is going to be supported for another 6 months or so (for a total of about 18 months).
As an example, LEAP 15 was released at the end of may, but LEAP 42.3 is still supported.
The minor release cycle used to be about 18 months in the past though.
Anyway as far as i can understand it OpenSUSE has multiple release cycles for different parts of the system.
No, the release cycle is that described above AFAIK, although basic components and some tool chains might not receive major upgrades to ensure stability and compatibility between minor system releases.
The kernel seems to be upgraded every 2 years
AFAIK the developers try to ensure ABI stability through the point releases, or for about 3 yrs starting with LEAP15.
This might rule out a major upgrade to the kernel; but please be aware that the “4.4.x” kernel for LEAP 42.3 included some (mainly graphics) backports from 4.9.x, so it is not entirely true that it was not upgraded for 2 years.
The base system since opensuse leap 15 from now on will follow SUSE will get upgrades every 3 years (or 2?)
SUSE try to ensure system stability for at least 3yrs (one major release and 2 Service Packs); this might mean ABI stability in the kernel and no major change to basic components.
For instance, SUSE15 changed suse-firewall to firewalld in SLES/SLED15: you bet to still find firewalld 3 years from now, so you can retain all your firewall configs for at least 3 yrs. This doesn’t necessarily mean that firewalld will not be upgraded.
And the packages will get upgrades every 1 year.
Userland packages will not receive major upgrades within a point release unless there is a compelling reason to do that. But within the lifespan of a point release you still find all package versions released in the update channel and you may choose whichever version you like (at your own risk, of course).
I must say this looks quite complicated and im wondering about the stability of the whole system with all those moving parts
I might agree that it looks complicated, but I can assure you that everything is done to ensure optimal system stability.
Then if you like “the bleeding edge”, choose Tumbleweed, where you get a complete new release twice a week or so, but also the occasional glitches and incompatibilities that come with its rolling model.
Disclaimer: all of the above is the understanding of a common (although loyal) user. Those with insider knowledge of the actual development process might have a different view