I just completed the Milestone Reward for collecting 500 powers for 5-Star Heroes, which rewards 800 5* shards. Is the best use of these shards to level someone I already have championed? Or is it better to use them to level someone to Champion status?
It really depends on your personal preference or roster needs. In my opinion, applying 1.6 covers to a 5* character you haven't champed yet is more useful than using it for 1.6 champion levels.
Milestone rewards are one of the few ways to (relatively) quickly target multiple covers for a specific character.
So it's obviously useful for backfilling essential characters like SC if you happened to take a break during their release period. And for new players those milestones might be a way to speed up acquisition of key 5*s as they build their roster. Otherwise, it's not clear to me that there is an optimal strategy for using those shards (which has led me to stockpile those milestones personally)
I used milestones to target Kitty, Apocalypse and BRB but only because I had Shang-Chi fully covered from hoarding. Having meta characters to move into the 5* tier helps quite a lot.
Agreed with above that it depends on your goals.
Milestones greatly helped me when I returned from a 2yr retirement. I was able to finish 5* Half Thor, cover BRB and I'm almost done with Mighty Thor. Next I'll target Shang Chi.
It always depends...I used the 1000 5* shards from milestones yesterday on my level 472 Shang-Chi! I use him every day in PvE and PvE. He's my favourite character. But that was an even easier choice because I already had all 78 five-stars champed.