Waterfowl is largely a buy once cry once deal, with maybe the exception of motorized decoys which need to be replaced frequently and lost decoys from time to time.
Buy a dozen nice decoys like Dakota or Tanglefree flocked. Texas rigs are worth their weight in gold. You don’t need a mega spread to start.
Calls are cool because you’ll always have them. I still have my nearly 30 year old Avery call that is still a really nice option for quiet, tight calling. Don’t buy an expensive call until you master the cheaper, easier calls.
Shotgun chokes matter. Even if you’re shooting a basic gun, invest in a good aftermarket choke. You’ll be glad you did. Lube it up every season.
For clothing, buy a nice outerwear set to start, if you can. Sitka or First Lite are likely the best, then maybe Drake or Chene or something in that middle ground. Good outerwear with the base layers you already have will make a LOT of difference in your comfort and enjoyment. Wool is your friend. This is one of the only pursuits we do where concealment and camo actually matter for most situations.
Waders…just don’t buy the cheap stuff, as you’ll be lucky to get a season. Plenty of middle ground and yes, fishing waders like Simms work great. Sitka are very expensive and has had durability issues but generally stand behind their product. First Lite apparently has waders coming next year which are supposedly bombproof but will also probably be expensive . We’ll see. No wader lasts forever but cheap ones will ruin your hunt.