Do I need to leave free space on the series x ssd like a pc or does the os compensate by...

It will reserve the free space needed. You do not have to do it manually.

Not really...

Xbox Series X games / upgrades / updates has to be installed onto the Series X internal or expansion card SSD,

You can have Xbox One games on your external hard drive that will get an Series X upgrade but the games will need to be transferred to the Series X and then the Series X upgrade will need to be downloaded onto the Series X SSD,

As an example...

I have Marvel Avengers on my external hard drive that will get the Series X upgrade and I will transfer the game over to the Series X SSD ready to download and update the game to the Series X version when available.

No, I did not forget about that, was into buying an extra 1 TB SSD also for Series S/X either way.

But thought it would be ok with Series S regarding power and memory, not targeting 4k.

My installed 70 games today on Xbox One use 1.4 TB(3 TB free), and this is no 4k content that might be forced on you if getting Xbox Series consoles just using disk space. So a couple of downsides going for next gen.

And looking at SSD recently I learned about server grade SSD, that has way longer lifespan - not sure what Series X offer here - you might need to replace in a couple of years.

Only a dozen digital games installed among those if going for Series S.

Now that I have 100/100 mbps on fiber I might do more gaming in future, will see. Still hesitate to download games over 20 GB.

p.s. not sure why notification emails does not work here, it does when buying from store.

Curious. The support site also doesn’t mention that this shouldn’t work. I’ll test other titles and let you know, but I’m sure Forza is the same.

Excerpt from Xbox series x FAQ

"Note Games optimised for Xbox Series X|S need to be played from the Xbox Series X|S internal SSD or the Storage Expansion Card. "

Hi,

Your drive will work the same on Series X as it does on Xbox One and you won't need to format it.

However, Series X games or Xbox One games with Series X enhancements will not run from that drive and will either need to be installed internally or to an external SSD designed for Series X/S.

Your hard drive will be too slow for Series X games. They can be stored there, but not run from there.