None of the bonuses are Alliance based as far as I know. The list I've seen only includes solo and party bonuses. The few party bonuses you're going to see if you're randomly matched to a party can be obtained just as easily with 5 or 6 in the group as 8. The more lucrative party bonuses require you to be joining a pre-made that are going in prepared to catch specific fish. You're never going to get those with a random group.

You can get the mount even with only 2 spectral currents and 2 players leaving your party at the start. That's what happened to me last night on this character. I focused on maximizing my catch instead of worrying that others had left the group.

There's a good guide on reddit for ocean fishing. Give it a read so you can go in prepared and improve your chances.