Some thoughts on parties. I always have things prepared, usually the day before.
At least two cakes in the fridge, a few party sized food dishes, mixology drinks. This just helps with those last minute goals that get thrown in like 'have 4 sims eat', make a toast etc etc.
Be organized with a sim that you control to 'be' the chef. Begin the cooking goal the second the party starts and cancel any chatting with other people, there will be time for the chef to chat once they've finished cooking.
With a simple goal like 'have three sims dance/play a game' etc try to do that the second the party starts with your household sims. As soon as the goal is 'ticked off' cancel the game/dancing etc and get every family member working on the big goal which is usually something like talk about something 12 times.
Be prepared for your own sims to be exhausted etc. It doesn't matter! The parties aren't so long that they will starve or pass out etc. better to have all their needs topped up before the party but it's not always possible with work etc.
Try to keep the guest list small unless you are comfortable with chaos.
The pause button is your friend. Pause a lot and think about what each of your sims is going to do next. Un-pause, let that happen and hit the pause again. Rinse and repeat. This is about getting gold, not pretty game-play!
If you run a great party and your own sims are still fairly good re: needs... order up a second party straight after the guests leave from the first party. Just make sure you still have a stock pile of the basics (Cakes, party sized food, mixology drinks). Sometimes it's better to just grit your teeth and get it over and done with.
I never empty my fridge of food , drinks etc as I WANT my guests to help themselves! Often those eating goals are completed by my guest helping themselves so why stop that!
I have my 'chef' sim standing at the fridge so he can grab the chopping board (ingredients) immediately the party has started and he rarely gets stuck unable to open the fridge because of guests helping themselves.
If guest sims are being annoying... get one of your own sims to begin dancing then click on (all/some of) the guests one by one and 'invite them to dance'. Gets there fun up and keeps them out your way to do whatever it is that you need to do! If you then suddenly need the guests to eat or chat etc, just do your Watcher thing and turn the radio off! Sims can't dance if there's no music.
Once you've got the gold medal don't feel bad about ending the party early, the sims don't care and that can allow you the time to get two parties done back to back... or to get your sims to bed in time for work the next day.
By basically doing these simple steps all my parties are gold. (In my legacy game too) I've only missed one gold medal and that was due to a game glitch in the Black and White party where sims are actually unable to 'listen' to a piano. So I know it works.
Hope this helps someone!
Happy partying.