Check out some third-party options if you wanna save cash for the wedding gift. These have TTL which basically means the camera and flash talk to each other to set the brightness automatically.

• Yongnuo YN568EX III for Nikon - solid power for about $90.
• Neewer VK750 II i-TTL Flash - very basic but gets it done for under $60. Ngl, just bouncing the flash off a white ceiling makes a massive difference compared to that tiny pop-up one. You wont get those ghost faces anymore.

Honestly, shooting a wedding is a huge task for a beginner, so you might want to consider something that wont break the bank while you are still learning. Since you have a Nikon D3500 24.2MP DSLR, you should probably avoid those massive pro flashes because theyll make the camera feel really top-heavy. I would suggest looking at the Godox TT350N Mini TTL Speedlight for Nikon. It is usually around $85, which stays well within your budget. Be careful with the manual-only flashes you see for $30 online. Since you're new to this, make sure to get a flash with TTL capability, which is basically auto-exposure for your light. It helps prevent those ghost looks by communicating with your camera. One warning though: the TT350N uses AA batteries. You should definitely pick up some Panasonic Eneloop Rechargeable AA Batteries 4-Pack to ensure the flash cycles fast enough for the ceremony. Dont want to miss a shot because the flash is still charging...

Building on the earlier suggestion about weight, you definitely want to keep things light or your arm will be dead by the cake cutting. Quick question tho, is the wedding venue gonna be super dark or outdoors? I would suggest these:

• Godox TT350N Mini TTL Speedlight for Nikon
• Nikon SB-300 AF Speedlight The Godox is better value, just be careful with recycle times if you fire fast.

Seconded!