For those interested in the history of the mini, a couple of years ago there was an interview with some Apple people and they revealed that Apple was planning on discontinuing the mini after introducing the $329 iPad 5th gen in 2017, because they thought nobody would buy the mini anymore at $399 (that was the then current price once every storage tier other than the 128GB had been removed) now that it was no longer the budget model.
However they noticed that people kept buying a 2015 device with a 2014 chip even after the budget iPad was introduced, and it did not really affect the sales of the mini 4, so people were buying the mini because of the size, not the price.
So instead of discontinuing it in 2018, they decided to make a new version together with the planned return of the air (which, at $500, would fill the gap created by the increased price of the pro to $800 and the now decreased price of the standard iPad) with the same specs but at $100 less, which was the same price as the mini 4 (but now at 64GB instead of 128GB for the mini 4). And in Spring 2019 the mini 5 was released, after 3.5 years from the mini 4 and a jump in chip of 5 generations.
From then on they seem to have moved to a difference rhythm, with both being upgraded every 2 years but the air first then the mini the following year.