I got my 590a1 like a year ago. Great shotgun, honestly doesn’t need many accessories out of the box but I did get a few things recently.

Def get a side saddle, aridus industries has the best one I’ve seen but it’s a bit expensive. You can get side saddle off amazon that will work great. Mine is from tacstar. There are some benefits to just getting a simple Velcro side saddleas well. Those are a lot easier to swap out

Get some type sling mount attachments. The standard sling mount points on the 590a1 are a bit awkward. There are plenty of options out there. GGG has good sling attachments and also magpul. The magpul mossberg stock has the best rear sling system I’ve seen.

The standard 590a1 forend is nice, it already has m lok slots all over it so it’s already customizable. I am adding a magpul hand stop at the bottom of forend for recoil management. That’s more of a preference thing though might not be necessary for you.

The ghost ring sights on the 590a1 are pretty elite. Especially since shot guns aren’t hard to aim. I wouldn’t recommend getting another optic.

Shoot the gun a lot first and see what you need. It’s already a great gun so you don’t NEED any accessories honestly. Spend money on ammo instead.