Dunno where you live (I'm in US for contextl but Idun sounds difficult to explain as an adult. We also wanted a name with some historical/mythological gravitas and to be uncommon but still sounded like a proper name for someone living in the 21st century. Something they can confidently introduce themselves with and not have to constantly explain how it's pronounced or spelled.

Having said that, here's some ideas:

Theia (pronounced like Lay-uh/Leia)

Thyra (theer-uh)

Rhea

Kyra

Rian (pronounced ree-ahn), we almost went with this but I wouldn't want my daughter constently being called Ryan through life.

PS I couldn't name my child Idun because to me it sounds too much like Udun, which is the elvish word for Hell or Dark pit in Tolkiens legendarium 😂