Buying the Antiquated Brionac is not a bad choice - even if you had been clearing A4S from the very beginning, you would still not have any guarantee of the Gordian spear for 8 weeks. I wouldn't worry about it not being the absolute BIS - it's the best you can get for now. If you've just got the spear this week though, I'd recommend getting the ring this coming reset week then the chest right after so that you can have an item a week with your esoterics for a few weeks.