I managed to score a closed box GoTrax Transit for half the retail price so I’m wondering what else I should get for it.
What’s a good bike lock so I don’t have to worry about it getting stolen? Are those U locks still easily pickable with a pen? Any cheap options out there for a rear rack and saddle bags? Lights? idk, what do I need so that I can keep it maintained? I’m on a tight budget so I’m looking for the best deals that I can get by with. I want to be able to use it get groceries and run local errands.
Also, do you take your batteries inside stores with you? I saw a lot of bikes on facebook marketplace with their batteries missing because they were supposedly stolen. Makes me think I should make a custom bag for it so I can carry mine around.
I do wish I had gotten one earlier, it’s a lot of fun. A little too much fun. I worry this is going to unlock a yearning for a motorcycle. 
Congrats! That looks like a solid basic model for getting into them.
With it being a cheap bike, my main concern would be vibrational stress on the frame and components. You’re going to want a multitool that fits every bolt on the bike. Tighten everything between rides.
Cheap tyres have poor grip and are easy to puncture. You’ll get a lot of benefit on bad terrain by upgrading to Schwalbe tyres with a sealed inner tube. I use Schwalbe Marathon Plus on my commuter bike, Schwalbe Pick-up on my cargo bike. Inside I use a sealant called Slime.
Pannier bags are your best option for cargo, but with that frame and size of battery you’ll want to keep it light with 2-4 bags of groceries. A milk box on the back would also work well and can be combined with side panniers.
You can’t spend enough on your helmet. I also use a reinforced motorcycle jacket, reinforced crash gloves, and when it’s cold out I use heated gloves and a balaclava. At 32kmh you’re basically falling off a one floor building and ebikes feel deceptively slow. Even if you splurge, the cost of your safety gear will be less than the time/opportunity/financial cost of breaking any bone.
Security comes from obscurity. Every lock can be broken and what you’re buying with a better one is the number of minutes it takes. I use a kryptonite ulock with a cable lock weaved through the frame as obnoxiously as possible. I always park near better bikes or ones that have worse locks, always in the most public place possible. The goal is to just make my bike the most inconvenient one they could steal for the risk.
They sell cheap fire bags for batteries. I use this one (https://www.amazon.com/Zeee-Fireproof-Charging-Explosionproof-Capatity/dp/B0BPSDY844/) for charging safety and transporting them outside of the bike. If I parked outside overnight I’d bring the battery inside for safety and temperature regulation, covering the empty port so water doesn’t rust the mount.
Whenever I think “wow a motorcycle/moped would be the more capable version of this”, I ask myself if I want to ride in traffic or not. The good thing about my ebike is the total freedom of movement over the most convenient and safest surface. It’s what allows me to ride almost every day of the year even if I’m just slowly crawling over ice. On a motorcycle I’m stuck paying for the privilege to use infrastructure I hate as the least safe thing on it.
Along these lines, a spare tube + patch kit, tire levers, and a frame pump are staple survival tools of road cycling. With enough riding, punctures are inevitable. A rechargeable battery-powered pump could be suitable too, but then you need to make sure it is charged and it is no longer something you can forget about until an emergency. With the exception of the frame pump, these are all things which can fit in a small bag below the saddle. These are all compromise / sacrificial tools though. You want to keep them on the bike at all times so you have them when you need them. They shouldn’t be anything too nice to be exposed to the elements or too precious to be stolen.
PRACTICE replacing a tube in the comfort of your home before needing to figure it out for the first time on the side of the road while it’s raining. Getting that second bead across the rim without pinching the tube can be a pain in the ass. It is a finicky process.
I would recommend also getting proper tools and fluids to keep at home for preventative maintenance, like a floor pump, a real hex key set with ball ends, whatever other wrenches you need. The multi-tool and frame pump are life-savers on the roadside, but miserable tools to use any longer than absolutely neccessary. Also a bottle of wet / dry chain oil (whichever is appropriate for your typical weather conditions), a lubricant like Tri Flo for the lever / brake / derailleur pivots and cable housings, and eventually some grease to keep the seat post from getting stuck in the seat tube. No WD-40 ever (unless you are trying to strip away the lubricant).
Also, 100% get yourself some eye protection. You are guaranteed to get hit in the eye by a bug at some point - sooner than you think. A pair of cheap shades will do fine.