Poland is a safe country; you don't have to worry about someone snatching your backpack or phone.
The city is currently renovating the large "Jeziorańskiego" roundabout, which separates the stadium from the city center. All tram traffic stops at this roundabout; after that, you have to transfer to a bus.
If you have a match ticket, bus and tram travel is free for the two hours before and after the match.
Taxis, Uber, and Bolt will be running, but the streets around the stadium may be partially closed, and getting to the stadium itself may be impossible (I'm not sure). And on matchday, the traffic are much higher. It's better to take the tram. Of course, it will be crowded, and people will be singing loudly about Lech. Sometimes the entire tram will jump. You don't have to sing or jump; it's not frowned upon. I almost never do it.
There are food and beverage options at the stadium. If beer is available, 0.4 liters of 3% beer will cost 18 PLN. Food includes popcorn, natchos and hot dogs (I don't remember the price, probably around 25 PLN). NOTE: You CAN bring your own snacks into the stadium, but you can't bring your own drinks – security will require you to leave them at the gate.
If you want to buy juice, water, or cola, you'll get it in an open bottle. I always bring a Coca-Cola cap from home, so if I buy a bottled drink, I can put the cap back on.
Officially, security can ask for ID, but I've never encountered this before.
There was also a question about whether it would be safe to wear your "colors" in Poznań.
Honestly, I don't know. It's possible that no one will say anything, but aggressive behavior could occur. Sorry, I can't be of any help on that.