Photoshop jpeg exports are < ~ 2mB. How to get the best jpeg quality?

You might find the following link useful. It’s an overview and comparision of how JPEG compression settings affect image quality and file size, and what JPEG does to get it that small. Although it’s written about Adobe Lightroom, it works mostly the same way in Photoshop.

An Analysis of Lightroom JPEG Export Quality Settings

The bottom line is that when JPEG Quality is about 75% to 85% of maximum, it should look essentially like the original and also difficult to distinguish from maximum. The link above shows that going all the way to maximum doesn’t really gain you any visible improvement, but the file size goes up dramatically. So, many of us export to JPEG set to 75% to 85% of maximum.

1MB for a 1080 by 1350 pixel image sounds about right.

What specifically is the quality loss that you are seeing? Is it sharpness, color, tone, or something else?

Instagrams max size is 1080px on the long side, as yoo are uploading 1350px IG will be applying its own compression.

The whole point of the jpg format is to produce small files – small enough to post online, or attach to emails.

To reduce the file size, it uses lossy compression which will affect quality. At the higher quality settings, this loss of quality may not be immediately visible, but lots of information has been thrown away in the process, even at maximum quality.

Image content  has a huge influence on the the file size of a jpg, so file size  cannot be used as an indicator of image quality.

Lots of sharp, busy detail (or noise) will result in a large file, smooth, flat, or out of focus areas will result in a small file.

See this post by @D Fosse for illustrations.

When I upload to Instagram, the quality is no where near the original quality

Do you by "the original" mean the raw file, or the exported jpg viewed on your computer?

If it's the former, the jpg will be nowhere near the quality of the original, it has been compressed, and the pixel dimensions are much smaller.

If it's the latter, it could be that Instagram is applying it's own compression to images. I don't use Instagram, so I can't say for sure.

Another possibility is that the image has been scaled in the web browser, which may cause loss of sharpness.

Browser scaling is unavoidable, practically every website will scale images to fit.

A third possibility is that you're viewing the image on a Retina/high resolution screen. When web browsers detect this kind of screen, they will enlarge images to 200%, otherwise they would appear very small.

Photoshop and Lightroom will not do this scaling. They have to display correctly, using one screen pixel to display one image pixel at 100% view.