Explain a DOB principle

Greetings...

I somewhat understand the F ratio---at least I understand how it's arrived
at. I also understand that higher numbers are smaller fields of view...

So, a Dob that's F/4 vs F/5 means what? I have an f/4.9 8" reflector and
an 8" f/4.5 reflector. The f/4.5 is more difficult to collimate...but I
don't see much difference other than that.

So, I'm looking at an 16" f/5.8 Dob. That seems ideal---am I right? A
little longer fl would possibly be a bit sharper on planets? And the truss
wouldn't be 20' long!!!!

Am I thinking right? Why do most manufactures only offer fast Dobs?

Doink