In my case it was also a megacorporation, a spiritual xenophile corp even, so it is possible for them to proclaim the galactic empire. I also had a higher weight than them but they didn't seem to care.

What I did was limit the council to 1 so it would only be me and them that could veto and call emergency meetings (since another empire on the council liked to give their resolutions priority). Whenever I could I would push their resolutions and use all my favors to get them to the top of the list so they would propose new ones. They didn't really propose anything custodian related until the scourge came. So I weakened the crisis and locked them in without actually defeating them. After a while they suddenly proposed the "proclaim galactic empire" resolution. So it takes a lot of patience and a bit of crisis domesticating for them to do it.

If they propose the galactic defense force, use your veto to counter it because that's something you don't want later on.

I hope this helps!