I think you are right that FSU is planning to leave. However, I think it is still open if they actually will. If they are stuck with a $500million plus buyout and will make less money per year for the next 5-6 years, they might decide it isn't feasible. The last FSU president and athletic administrator made statements about being happy in the ACC. The ACC has a quote in their amended filing quoting a former FSU Board of Trustees member who said that the GOR was the best thing for FSU and the ACC because nobody can afford to leave the conference with it in place. He was on the Board of Trustees until 2020. It is possible that before FSU leaves, there are new BOT members, a new president, and a new AD. IF there is a new AD, FSU might decide not to leave the ACC yet. A long winded way of saying that even though FSU is fighting hard right now to leave, we don't know what will happen in the future.
I do not buy that FSU has a home waiting for them. The Big10 is very unlikely to pay a full media payout until the end of the current contract. They didn't for Oregon. ESPN has reportedly rejected requests from SEC for more money. Why would ESPN pay a full SEC share for FSU, when they already have FSU content at a lower price? Why would either the SEC or Big10 have actual on-the-record discussions and offers when it opens them up to lawsuits for tortious interference? The only argument I have heard about FSU actually having an offer in hand, is that they wouldn't be crazy enough to file this lawsuit if they didn't. However, if you read their initial filing, it does not read like a well thought out legal action. It reads more like a fanboy forum post that goes into emotional detail about how FSU was screwed over. I think it is more likely that FSU believes that if they make things difficult enough, that the ACC will cave in and let them have everything they want.
Also, there are programs in the ACC that are more attractive to the Big10. There has been a lot of speculation that both the Big10 and SEC want UNC and UVA. To get into Florida, I think the Big10 would rather have Miami as a member than FSU. Miami shows up higher on most of the "brand" rankings that I have seen. Miami is an AAU member. Both are in the state of Florida. IF Miami were to join the Big10, there isn't a need for another member from Florida. I am pretty certain that there isn't an immediate home with a higher media payout than they are currently getting from the ACC. I think it is only questionable whether FSU would find a home in either of the top 2 conferences at all if the ACC collapses.
I think the best option for FSU to get what they want would be for the NCAA to fall apart or split, and cause a complete realignment of college sports and conferences. If that happens, the FSU would need this lawsuit.