Business class flight just 2 weeks ago and will be my regular route, never once was I told the loo was free, got changed in the very clean toilet, took my make up/lenses out/put hair up at my seat, then about an hour to landing got changed in loo repeat with make up etc in seat. Nothing unusual about any of that, but I have my routine and it works for me.
To the op, congratulations and how exciting the flight, holiday and celebrations will be, just be comfortable and have what you need to hand, as I usually am coming from a cold place to a hot and humid one I will have a change of clothes for the other end, over the years I have always put the change of clothes in a large ziplock bag clothes folded and air squeezed out, flat easy to pack in hand luggage and they are crease free to change into.
You may or may not get an amenity bag dependent on airline/time of flight, but you will find toothbrushes/paste maybe hand cream etc in the loo, again dependant on airline. You will have a pillow and be offered a blanket, or like Emirates have a topper for your seat.
And I wore a mostly white shirt (not crisp!) with black loose pants with a pair of Geox trainers, then changed top and put a pair of Birkenstocks on, at the other end job done!
Just enjoy it and dont overthink it!