can you buy more of the same boost
No, you can't. I also doubt that it would be a good idea, and even more that it would be implemented. It would give an insane maximum boost. It may also make some GBs superfluous; if the tavern boost alone is sufficient, why would you need GB boosts?
how about expansions
first: 1 million
second: 2 million
third: 3 million
A good idea, unlikely to be implemented. I suspect one of the reasons behind the tavern is to give people another reason for logging in frequently. More table expansions will reduce that effect. We're probably lucky to have as many as we do. Also, it will merely postpone the problem; once having those boosts, the players will earn even more silver they can't use.
How about giving the guilds some use for silver? For instance communal attack boost added to the individual boosts, purchase of guild goods for silver, or buy "plunder insurance" for members (plundered resources reimbursed).
Also, an individual one I'm pretty sure will be popular enough to drain many coffers: A boost to goods production. I don't think it would have to be very high, in order to become very popular, especially after just having progressed an era.