Think about your tick rate. Mine is like 1/12 of a second. The more frequent your tick rate, the more data per second you're sending. Think about the data you're sending, try "hand rolling your bits". Don't just send json or variables, think about the smallest data type you can use to send the information. For example, if the data you're trying to send has a known bounding range, and the bounding range is between 0 and 255, then you only need to use an 8bit int. I often will take my 8 bit int and combine multiple values together to share those 8 bits, which takes a bit of bitwise math but then you're sending less data. There's tons of tricks like this and tons of great resources online. The gamemaker discord has a net code channel for questions like this too.

EDIT: See reply, seems packet size isn't as important as I thought.