I’m not familiar with the math for gold and bronze chests but I would think that you could, but someone else will have a better answer in relation to the discount.
What you can do to be safe is, get as far as you can on the discount, if not all of it and just save your sigil and rubies till the end of the season and make your decision then. That way you won’t have any chance to waste rubies.
Also 25k sigil gain is relatively conservative. I’m not as active as you and don’t do quests and our team never gets the 1k team sigil prize and I accrued 21k sigil while nearly missing a couple events. So do the real math that you know you’ll get and make a decision from there.