LC connectors are smaller and what the actual SFP+ modules typically have. If you want to run a link with just one fiber, you need BiDi optics.

FS does custom multi-fiber cable assemblies too (beyond the duplex patches which is basically the standard), and they can also include pull eyes on them if that’d be helpful.

Single mode is a good choice, common wisdom used to be multimode for short runs but the single mode stuff is not much more expensive and the standard 10km optics will likely brute force the signal over any mistakes like cable kinks or dirt on the connectors.

> Guess, I'm back to CAT6 :-)

If you learned what you need for 10GbT you can learn what you need for 10GbLR. Which is:

LC connector, PC or UPC, duplex, OS1 or OS2, and SFP+ modules saying "LR".

Any of the following is wrong: SC, FC, LSH, E2000, ST, APC, simplex, OM[1-5], "SR" or "ER" SFPs.

And that's short enough.