well you are lucky, i not own nothing legendarys. only rares

RNG is RNG, nothing you can do will change the quality of the champs you pull. 

With that said if I was gonna be buying any shards it would only be Primals and Voids. 

Unless you can comfortbaly afford to be a whale spender, don't buy shards at all.

The monthly gem packs are by far the best value, and maybe the occasional forge pass.

IMO   Voids  as  a  newer  account  goes  .   So  many  void  epics  and  rares  are  useable on  a newer  account  even  when  you  do  not  land  a lego  but  voids  will  give  you  the  most  quality over  quantity    IMO

Even Forge Pass these days is probably only really good for early gamers.  Except maybe the Bolster one.