Photoshop JPEG saving options

One thing that is helpful to see what works is to use the "export as" menu. It shows you a before/after, with all the parameters updating in real time. The level of compression will depend greatly on the image. Lots of different colours will degrade greatly (I am simplifying here) if you dial down the "quality slider), big area of sky and water, where the colour variation is minimal will fare better at those low settings. It has to be seen to be understood.

When you see what your settings will be, use the standard "save as" JPEG to control the resolution as I don't think you can use 300 dpi in the "export as" menu (It's the new save for web!)
@Rob_Cullenlink is very informative!

You could argue that quality 12 defeats the whole purpose of saving to jpeg in the first place.

It's important to understand that jpeg compression is always destructive at any quality level. There is no such thing as  "maximum" quality with jpeg. If you really want maximum quality, don't use jpeg at all.

As long as the file will be somewhat degraded anyway, it's better to see it as a tradeoff, and to determine the optimal balance between file size and visual quality. And seen this way, the best "value for money" is often around 8 or 9. There is still very little immediately visible degradation, but the size reduction is substantial.

After all, the whole point of jpeg is to reduce file size. So you might as well maximize payoff.

I don't think "progressive" is used very much these days. That was for those extremely slow dial-up connections of the old days, so that you could at least see that there was an image there while you waited for it to load.

An informative site link-

JPG COMPRESSION EXPLAINED by Jeffrey Friedl

2000 is not a type of JPEG, confusing huh! JPEG 2000 is an entirely different format (invented by the JPEG people). It's not used on the web. Don't use it unless someone tells you they need it. 

JPG, JPEG, JPE are all different ways to name the same thing (normal JPEG).

The quality settings when you save are very important. A file can be terrible quality, or ten times the size you need. Experiment.