Could you please answer a few questions regarding this OS?
- Does nvOC have a "graphic" online interface like SMOS?
Dont know SMOS, but it has a telegram notification and the web interface looks like this :

- Is it possible to easy install new miners?
Almost all important miners are included, adding new ones is easy too
- Is there any "hidden" fee for using it?
No fee at all for nvOC, some miners like claymore has their own fee
- Does it have general "quality of life" features, like: restart with auto connect, auto restart miners, ...?
Yes, Auto restart miner and reboot rig if hash rate drops or gpu fails, timer reboot,...
- Does it update (or auto update toggle) video cards / OS?
It has auto update toggle for stable and beta
System updates are blocked to get more stable mining and will enforce by nvOC
- Is it possible to enforce overclocking for GPU as soon as the system restarts (auto)?
Yes you have many options for overclock and power. by individual GPU, by mining coins , all same ...