Прочитали would more accurately translate as "We read these magazines," meaning we finished it.
"We have read these magazines," implies Ми читали ці журнали, meaning that we read them in general, maybe some of the articles, or maybe just didn't finish with it.
Ми читали ці журнали to my eyes looks more like We were reading these magazines, when something happened. Ми прочитали ці журнали - we have read these magazines, and now we know what's in them. But I am not sure which one is better for English We read these magazines.
It can be both my way and your way, depending on the context. English doesn't have perfective/imperfective verbs, and Ukrainian doesn't have the present perfect or past perfect tenses, so those gaps are filled by context.
Your example, you're adding context with "...when something happened," so you have made it so. "Ми читали..." does mean "we were reading," past continuous tense. Now replace your context with mine in the sentence, and my example works too. Yes, "we have read these magazines previously." Now, "ми читали ці журнали раніше" carries the imperfective aspect, unspecifying when they were read or whether the magazines were read completely. Alternatively, "ми прочитали ці журнали раніше," carries the perfective aspect, meaning that we were done with these magazines previously.
Whereas, "ми прочитали ці журнали," means explicitly, "We read [past tense] these magazines. Done," AND "We've read [present perfect] these magazines. Completely."
Context is key. "We read [past tense]." "We have read [present perfect]." Both can mean either ми читали [imperfective aspect] or ми прочитали [perfective aspect], based on context.
There is no tense in Ukrainian, and there is no aspect in English (as a feature of the verb, it exists in context of course)
So either one can be translated as either one.
Прочитали can be "read", "have read", "have finished reading" and "finished reading" depending on the context.
Читали can be "read", "have read", "have been reading" or "were reading". Or even "finished reading" (meaning, "finished the reading", not "finished the book")
In short, the aspect in Ukrainian contains information not about the action itself, but about the end result (i.e. whether you finished the book, not whether you finished the reading) which the English verb does not. "Прочитали" means the reading was done in the past and that the book itself was finished, read until the last page while "читали" does not give that information. The English "has read" or "read" do not have any information in them whether the book was finished or not (the reading was for sure).
This is an interesting point. I hadn't thought about it that way before regarding tense vs aspect
"We read these magazines" can also indicate completion (since "read" is the conjugation for both present and past tense) and should be accepted. The auxiliary verb "to have" is not necessary to indicate completion in English.
"this magazines" is wrong. "Magazines" is plural, so it must be "these magazines"
Unnecessary confusion in presentation of aspect by choosing "read" as the example - in English this form can be present, past, or past participle.