If the maxed Eddies are also naturally 5 stars, sure. A natural 4 star be worth less than a natural 5 star (which will be worth less than a natural 3 star, etc), but you won't need a whole bunch of maxed out nat 5 Eddies to get a reward from the highest reward tier (in fact, that would be crazy overkill).
For clarity, "Natural 5 star" means that's that character would normally be earned as a 5 star from souls (or what we would put it at if it were collectable from a soul).
That character alone will get you one of the last sacrifice tiers, but to maximize it's value, level him up with XP shards or doing battles in the campaign to get that essence even higher!
If you have skill shards invested, a skill shard icon will appear on the character's icon when you're choosing which characters to sacrifice. You can also press and hold to confirm if that character has skill levels, if they have stat changes from talismans, etc (the same Character Info popup you'd see in things like the Pre-Battle screen).
Also, you cannot sacrifice anything you have locked, so once the feature comes out, this is probably a good opportunity to unlock all the dupes you'll be ready to sacrifice (or to lock up all your valuable characters).
Those achievements care about getting the characters, not the amount in your inventory. You're free to sell (or soon Sacrifice) characters you've evolved to 5 star without reducing your progress for the "Earn 5 star characters x100" and related achievements.





