I think there might be confusing things going in your wife's head, OP.
Firstly, she probably isn't feeling very sexy. You don't say how long ago she had her mastectomy (unless I missed that...?) But whenever it was, she is still obviously getting her head around it. Maybe she's still in pain. Maybe the scars feel awkward or ugly to her. Maybe they itch or feel irritating. So right now she doesn't feel sexy. But you do, and that's going to be problematic, even if it's no-one's fault. Maybe she feels guilty. Maybe she feels pressured. Maybe she feels stupid to feel the way she does.
(I know that when my mum had her mastectomy, which she really had not wanted, she showed me her scar often and wanted me to reassure her that it wasn't ugly, and that she wasn't ugly :( )
I think three things might help here: (a) Time, (b) Take the focus off her breasts for a while - not just when you have sex, but at other times. Tell how how sexy you find her brains, her sense of humour, her hair, her bum, whatever... Help her see what you have said here: that her breasts are only a tiny bit of what you love about her... And (c) Imagine how you'd feel if you'd had to have your penis removed. Seriously. Her breasts are every bit as important to her identity as that and possibly more so, since I guess your penis isn't quite as visible to other people! If you can empathise with what she's feeling, it might help you understand some of the complicated things going on in her head...
The other thing that jumps out at me is that your wife is caught up in other people's reactions and 'mixed messages' - not just yours, but across the board. She (and both of you) will probably have been through a whole gamut of feelings: fear, anger, relief, grief, etc... Meanwhile, she'll also be trying to 'manage' other people's reactions and emotions, and that adds to the confusion. For example, she'll be protecting your children from the fear, but simultaneously pretending that she is smiling at them dancing around with her prosthesis on their heads. Friends probably help her relax, but also pity her. Even you are giving a bit of a mixed message, simultaneously saying you think she's sexy as she is, but also saving for a breast reconstruction.
It's an almost-impossible balancing act, and it makes it hard for her to work out and work through her own feelings.
Of course you can't change what goes on in anyone's head. But you can reassure your wife that whatever she is feeling is OK. (The jargon is 'validating her emotions') If you can let her feel whatever she is feeling - accepting the bad as well as the good - it will help. :)