I know it's not exactly the most useful weapon at higher levels, but I absolutely love the way it looks, and other two-handed weapons swing way too slowly for me to really enjoy using them. So, bearing that in mind, I'm trying to figure out how to best use Ebony Blade. Do I have this straight?

Ebony Blade's damage is based on your One-Handed skill…
… but using it boosts your Two-Handed skill.

Its damage is only boosted by Fortify One-Handed enchantments, and its damage benefits from damage-increasing One-Handed perks (Armsman)…
… but extra effects, like decapitation, are based on Two-Handed perks (Devastating Blow)…?

Does that sound about right? And if you use this weapon yourself, do you wait until you have One-Handed up to 100 before you bother with this so you can get max damage out of it?

Correct.

There are some mods that make it more useful at high levels. For example, making it smithable and benefit from ebony smithing perk.

Thanks for the confirmation. I like the look and the swing of it enough to be willing to invest into two different perk trees just to make it useful (why not, if I'm going to massacre 6 followers for it anyway?), though the slog to get 100 in One-Handed is longer than I remember it being...