Please think long and hard about doing 'Backer only' updates at all, especially during the campaign. I absolutely refuse to back a kickstarter that has any 'backer only' updates during the pledge drive. Personally I would take that reward option out of all the tiers. A limited 'early bird' reduced cost digital copy of the game only, as someone else suggested, is a really good idea to get a burst of funds at the beginning.
Kickstarter games have become quite a gamble for backers with little to no recourse if things go wrong, even after a successful campaign. I believe transparency is really important. Since you will have a commercial release of the game your kickstarter page, even years after the completion of funding and delivery, becomes a form of information for anyone researching whether to buy your game. Information needs to be accessible.
Sorry if I sound harsh I've just seen so many kickstarter otomes and vn's flounder even after initial success. Also kickstarter has engaged in some union busting behaviour recently with some of its employees which has put a lot of people off supporting the company and made it even more difficult for the achievement of funding goals.
Anyway, Good luck and I really hope this works out as 'The Rose of Segunda' has become one of my favourite otome/vn's and I would love to play its sequel.