Even though I managed to earn a gold wedding my first time out, there were several things that could have gone better with a bit of advance planning.
Like others have said, it's best if bride and groom already live together, and you host the wedding on a day they both have off. I started the wedding in the morning, as soon as all need bars were full. And still...
The caterer didn't manage to cook a single thing- just left unmixed cake bowls and plates of food in progress. Good thing I already had 2 party platters of food in the fridge from leveling. I put them in separate areas so pathing wasn't an issue.
The tiered cake I made in advance *spoiled* as soon as I took it out of the fridge- even though it was supposed to be good for another 20 hours. As soon as I realized the cake had gone bad, I had the groom greet guests while the bride whipped up another tiered cake and placed a wedding topper. Even if you don't have level 5 gourmet cooking for the tiered cake, you can bake a white/chocolate cake at level 1, strawberry at level 4, blue confetti at level 6.
For some reason the bar needed to be cleaned as soon as the mixologist made a round for the party, and neither bride nor groom were able to get in there and clean it. The wedding party didn't last long enough for anyone to get sick from the dirty bar. (can they get sick in sims 4?)
The wedding arch does NOT put your couple into wedding attire, so before the wedding use a dresser to create formal wedding outfits and save them as formal 2. Then right before you start the wedding event, change them into the outfits you want them to wear. And, uh oops, the musician will not provide his own instrument, so it's a good thing I already had speakers playing music.
I wasn't sure if the guests would watch the ceremony without chairs, so thankfully you are able to go into build/buy mode after the event begins and toss a few chairs in the backyard next to the wedding arch. For some reason, everyone congregated in the bedroom by the laptop, so when it was time for the ceremony I just used a sledgehammer on the outside wall to eliminate pathing issues and get everyone there in time. Once you start the wedding with the arch, they will all make their way over to witness.
For a cute "wedding picture" you can take selfies at the arch.
In addition to planning the wedding, next time I will also hire a maid for the day, to clean up all the party mess.