This Q&A, explains the form vるにはvた. However, when speaking with some Japanese friends I noticed that they are more likely to use 読んだには読んだ instead of 読むには読んだ. Is that a spoken-language usage?
By the way, they perfectly understand what I want to mean if I use 読むには読んだ
読んだには読んだ and 読むには読んだ feel exactly the same to me. I personally feel the latter form is a bit more common, but they're both perfectly natural. Note that the first verb never takes ます when you are speaking politely.
Regarding formality, this には sounds a bit stiff. When talking with friends, 読むには読んだ is okay, but more casual wordings such as "読みはした" and "読むっちゃ読んだ" tend to be preferred.
