I think this depends on where your account is. Early when a lot of champions can change your account I think it's best to pull on any 2x event, skip 10x events, and possibly skip over the guaranteed because they usually run with no x2 or even summon event.

As you end up needing more specific champions as you progress 10x is still terrible but becomes more worth it.

For most accounts I think it's best to pull during fusions as you will triple up pretty much between the event, what you get from pulling, and the fusion at the end.

There have been some really good guaranteed champs but I feel it's mostly for end game hoarders or spenders, and even though the champion is good it probably doesn't chAnge your account anyways. I just did the 20 for elvia or who ever. Lots of shards I got the guaranteed and one other leggo. Not really worth.

In the beginning, x2 events are the event you want to pull your shards. Every legendary is potentially and probably a new addition to your rooster.

x10 Events for non-void legendaries....just forget about them. That is my personal view, but with the number of legendaries in game, a x10 equals something of about a 6% chance to get the one depicted int he x10 IF you get a legendary...void is slightly better but stil not guaranteed

lategame/endgame: save a substatial amount of shards for guaranteed legendaries

I think early on it's the 2x events.  You need champs to fill broad rolls.  At that stage almost any pull will improve your account.

As you build a roster, it's more about filling gaps/needs, and points for summoning/chase events.  So you'll try to maximize that aspect.  Paying attention to where you are in pity.  Etc

Near the end it's pretty much garanteed.  I've had a goal of holding on to a baseline of 15 sacreds so just this past week had to pass on the garanteed sylvan leggo.  Upping that to 20.

For voids, per your question, garanteed epics usually require 40 shards or so, and garanteed voud leggos usually 80 or 90 shards.  I would not consider holding shards for garanteed champs until you have a stable supply of shards. Either nailing unm or deep pockets.