The way WoW does it is that the achievement is marked as account bound completed but the subtext under the header will be greyed out for that character who still has it uncompleted. So the uncompleted achievements titles across your account are dark grey, the account completed ones are gold. Of those account completed you click on and the subtext (for quests it’s the name of each storyline) will either be gold or grey depending on if you’ve completed it on that character. On the gold achievements there’s also a marker to the side showing the number of achievement points gained and scrolling over that also shows what characters it’s completed on. It’s pretty simple to use and I wouldn’t mind a similar system in ESO.