I would wait for a 2x legend or 10x individual boost on your shards.

So far you must have around 600 points from your 40 ancients. Best case with your 8 shards is you pull all legos and can just scrape the 2 legendary tomes, most likely you will hit 4-5 rares, 2-3 epics and maybe 1 lego. So maybe another 1500 points which means you still miss the lego tomes and have 350 energy, 24hr xp boost, epic tome and a 5* chicken.

Either way it is a gamble, just depends what you are pushing for. I am waiting for the next counter attack champ boost having still missing out during the last one with Valkyrie in it.

What you say Daering, is true for any boosted summoning events using ancients... but if you want a specific legendary champ, then 10x is better than 2x boost for any champ. 

Either event results in plenty of food for your ranking up champions though.

I get a lot of mystery shareds every day.  I am thinking of saving some each day until I have 1-2,000 on hand.  Then the next summons event I can crack shards and have 2-3,000 points.  Thats enough to get me into some of the better rewards.  Maybe, I finally get my Coldheart with some luck.