Disguise a belly button

It may be something that you notice, but others would just see you looking lovely in your dress. Maybe try taking pictures in a few different lights to see how it looks again before you spend lots on new underwear etc - it may not be a problem on most lighting settings. Could you try your dress wearing one of the circular white or skin coloured nipple covers on your bellybutton? They are cheap, easy to source, discrete, and mean you don't have to change your planned underwear - plus you could just wear it if feeling a bit conscious for formal photos then take it off later and just enjoy your day. If your dress has capacity you could try a low back slip or underskirt but that may add a lot of cost to solve something that no one else would notice. There may also be a sash/belt you find that complements your look. Good luck - I'm sure you will look lovely!