Just bought a sub2000 9mm what upgrades are recommended
Take a look at MCarbo products. https://www.mcarbo.com/ They have several interesting upgrades worth looking at. I went with the muzzle brake, the Bolt Tube Cover, buffer, and recoiless Chargong handle....so far. The swing out Optic mount and a rear folding tactical sight will follow as funds permit.
I found the existing rear sight virtually useless, unless you like standing on your head to shoot. An Optic solves all of that. I will opt for the open notch rear sight as opposed to a preep sight as a backuo to the Optic.
The iron sights are hard to use, so a dot sight is a big help. Not having used a dot sight before I bought a $25 one on eBay. It works ok but is not as bright as I want. Yesterday I ordered a Sightmark red/green dot that's on sale for $85 at Midwayusa. It is a quick release mount so I can just detach it from the top rail and attach it to the bottom rail when I want to fold the gun.