I use "someday" all the time when I want to say いつか. But recently, I realized that some native English speakers use "sometime" to mean いつか. What's the difference between them? "Sometime" sounds like things are more likely to happen in the future? I am unsure.
In my opinion, I think we use 'sometime' for the near future, and as you suggested, it sounds like things are more likely to happen in the future .
"Sometime" has two parts of speech, a noun and an adverb.
For example.
(1) I lived in Tokyo sometime ago.(=before)
(2) He will visit my house sometime next week.
(3) She stayed in Yokohama (for) some time.(しばらくの間)
(4)I have some time.(時間が少しある)
As a noun, we use it with two words 'some time'.
And as for 'someday', we use it for the unclear future.
Things might happen or might not happen.
And it is only an adverb.
For example,
I want to go around the world someday.