Speaking from an en_UK POV 'a myriad of' is how I'd normally use it; the poetic adjectival form is nice in poetry, but doesn't feel quite right to me in normal language.
Whether I'm correct to feel like this is left as an exercise to the reader.
When I lived in London I noticed lots of people got it wrong. If you think of it as a synonym for 'many' rather than 'lots' it's easier to slot into sentences correctly.
Whether I'm correct to feel like this is left as an exercise to the reader.