What is your perfect/ideal focal length?

1400

With pixel size of 3.7µ i would say 1200-1400

Roundabout answer: whatever focal length gives me a pixel scale that is appropriate for my seeing.

So like ~1000mm for 3.76 micron pixels.

From my backyard I use approx.  300, 600, 1000 and 2500 mm focal length telescopes.

If I was allowed to have only one, I would keep the 2500 mm (CDK14) with the FF mono camera.

1200, like Andrea, with 3.76um pixels

if I decided to sell my 2600mm-p and go OSC, I'd get the 2400mc-p (5.9um pixels) with that FL

I like 1000mm, with FF sensor on the TOA-130. Can image smaller galaxies and larger nebulae relatively well and offers plenty of versatility.

10 meters, give or take

1000 mm to 1250 mm with my 2600mm camera.

1200mm with 3.75 micron pixels.

I probably should have clarified, I meant full frame just from a framing perspective. Ignore pixel scale etc.

Not really. It's just a fact that at around 300-400mm there is a limited choice of targets and there's nothing worse than pushing things by trying to image small objects unless you're going for context. The other reason why I think short focal lengths especially with big or small sensors is popular is because it's relatively it's easy. Guiding and seeing can be pretty crap without effecting your image at all. You'll still get something that  looks amazing. Also setups can be small and light, great for mobile shooting.

I think you'll find that at least some people who have been in this hobby for awhile will eventually gravitate towards longer focal lengths. Sure it's more challenging but there is so much more to photograph. You have a chance of coming up with some unique images. For instance, I'd rather image a tiny detail in M-42 than make the picture that everyone else has done to death. I know some people won't like what I just said so I want to be clear that I'm not belittling people who do shoot in the shorter range. I fully realize that fighting tilt with a FF sensor can be a pitched battle all into it's self. This is strictly my personal opinion as an answer to your question.

On mosaics, yes it's an option but with integration times stretching into the 20 hour range or higher for single images becoming common, it's hard to commit to a mosaic project that might take 4 times that. The other thing I think people don't consider enough is how your final image is going to be presented. Most people are going to see it on a laptop or desktop screen (don't even get me started on phones!). At that scale, you won't see the full resolution without pixel peeping. Take a image taken with a large sensor and view it at the same pixel pitch as you screen and you'll see what I mean.