Hi! I’m not a dentistry student but I can attempt to answer some of the questions for you!
Accommodation: I lived in Senghennydd Hall for my first year, I wouldn’t recommend it to you as it’s quite far from dentistry, however on the whole all of the rooms I’ve seen are very good, the maintenance however is not the best, I’ve had to wait around a lot for repairs and things in my room, plus a few times the maintenance staff have walked into my room while I was in there without knocking first which is a little awkward! In terms of choosing, I’d say the best thing to do would be to consider distance to your buildings first and foremost, because you will regret choosing a far away residence when you have to get up for 9am lectures!! It’s easy to find the postcodes of the residences on the Uni website and you can have a look what’s around on google maps, or see what the distance is from certain facilities.
Best/worst things about the city: The best thing is the size and convenience of the city! For a capital city it’s on the smaller side, and the city itself is very compact so it’s fairly easy to get from one place to another. For me, it’s not too big or crowded but it still has plenty of shops and things to do! The worst thing personally is definitely the distance from my family home, but might be different for you. I’d say in general maybe the lack of a big supermarket in the main area of the city as it can be hard to find certain groceries in the smaller shops.
Accommodation after first year: You’re pretty much on your own with this, there is a housing fair towards the end of first year so you can look at your options, but most people find a house with some friends. I believe you can stay in Uni dorms still but you get whatever’s left, you don’t really get to choose where you stay or you don’t get your first choice at least. Finding a house is pretty easy, there’s a housing place in the student union too if you don’t want to use external landlords or whatever.
Student union: The student union is pretty good, I’d imagine it’s quite standard for student unions across most UK unis, there’s student support and advice centres, I don’t think they’re the best things ever but they’re good if you want basic information about things. The club rooms in the student union are okay, they get ridiculously busy and crowded though especially on specific nights, so it depends what atmosphere you’re looking for. There’s a few shops, a co-op, a barbers, and a computer repair shop for example.