If you're happy with your current breast size, I would recommend wearing a comfortable, supportive bra even at night when you sleep. It sounds like you should also consider downsizing your breast implants. Best wishes to you!
If you're more comfortable sleeping in a bra then that's fine. In fact some companies market a "Sleep Bra" online which several of my patients have commented positively about. There have been several studies which show wearing or not wearing a bra makes no difference in whether women's breasts sag or not - pregnancy, weight gain and/or loss, genetics and the normal aging process are all factors.
I think a comfortable bra is best. I do not think a bra will prevent the breasts from sagging long term.
Some soft support sounds like it is making you more comfortable at night and that is important. Underwire bras may not fit right as you lay in different positions, but the soft support should be fine. Make sure that the support is not too tight.