Birds don't often know that they are looking at a reflection, they think they are looking at another bird. If a bird becomes too bonded to their mirror, any other bird that doesn't look like them, nor act like them, may get attacked. Some birds can form such a strong bond to their mirror self that the only reason they leave the mirror is to get food and water. And even in a flock situation, you may find that the birds pay more attention to their reflections than they do to themselves, not to mention birds possibly bonding closer to their mirror friend than to the humans.
It can work having mirrors for birds, but I just don't recommend it. I've seen birds (particularly little ones) become aggressive around their mirror and then the humans have difficulty handling the birds. Remove the mirror and the aggression goes away.
Have you ever felt that greasy, dusty feeling you get around a stove if you don't clean and wipe down surfaces around the stove regularly? I actually handled a cockatiel that felt that way. He hadn't been bathed in who knows how many years, lived in a closet for I don't know how long, and was so bonded to his mirror (then covered with grime) that he only left it for food and water. He didn't move around his cage, he didn't chirp, didn't sing... just sat there, next to his cage. It was sad.
As far as flock situation.... I know someone who had/has (it was some years ago) a cockatiel flock and one bird was a loner. All he did most of the time was sit next to his mirror rather than interacting with the rest of the flock. I asked about removing the mirror, and it was taken away. Well, this tiel started moving around the cage more and interacting with the rest of the flock.
Budgies can be little beavers, so I really recommend chewable toys for them - as well as shiny toys and bead like toys.
I was given some colored popsicle sticks with slots in them so that they could be attached to each other. (don't know where to get them!) When I had budgies, I gave them a "box" made out of them (also gave it to the tiels - a more complex box) that they enjoyed chewing up and sitting on. Once they finished it off, I'd make a new one.
I also had an acrylic swing for the budgies. They enjoyed sitting on it and playing with the bell.
Also had a bead, bell and popsicle stick toy for them.
In years past, I had a budgie that *loved* the "Olympic Rings" toy and a different flock that *loved* a hamster wheel! (Casey, my first tiel loved it, too, so I had to get her a ferret wheel that she could actually fit into! LOL - although I no longer have wheels)