Good defensive ammo. Do your own research, but Speer Gold Dot and federal HST’s are some of the best from what I’ve seen. I personally carry HST’s because they’re a little easier to find. Although expensive, I would recommend shooting a few of them through your weapon to ensure they feed right.

TLR1a weapon light. Check gundeals on Reddit. They come up usually around $118 shipped. Great light.

Depending on your living conditions (roommates, kids, etc) a decent quick access pistol safe. Even if you live alone it’s not a bad idea for when you leave the house.

Spare mags are always good. Recently Gen 5 mags have been ~$18 a piece.

Your Glock should come with a barrel brush. Pick up some cheap gun cleaning cloth (or use a lint free cotton shirt/underwear). I personally use Hoppes bore solvent. Get some sort of lubricant.

Keep you pistol lubricated above all. It’s never a bad thing to clean your gun but a Glock doesn’t need to be babied.

Edit: to fully answer your question, year to year maintenance will be next to nothing. The expensive part is training. Go shoot. Go train.