The 300-800 isn't really a possibility. It is $8k - more than the 300/2.8 II, has no IS, is blown away by even the I version of the Canon teles and Sigma has a horrible reputation for quality - especially in their telephotos.
The 300/2.8 version I is a possibility, but at this point I am taking a different direction. My wife will likely not approve the budget for that lens because she knows me too well. She knows it will only be a temporary stopgap until I can afford something better whereas if she approves of the expense she won't want to hear about another telephoto for a very long time.
The other argument from my wife's perspective (and she has a point here) is she already agreed to let me buy a 600/4 II if the novel I just finished makes enough to pay for it. Therefore her opinion is that I am cheating by asking for a telephoto now - even if it is a much cheaper one.
I will likely invest the money into improving my drop photography solution instead, where I can have a much bigger impact given the budget.