There is no easy, cheap or free way to record audio directly into an iPad/iPhone.
Pretty sure the Griffin iMac will NOT work with any iDevices.
Not sure about all of the iPad Pro models, but the 12.9 inch iPad Pro models have a throughput, via the Lightning connector, equivalent to USB 3/C speeds.
Not sure about the speed of the lightning connector on the 10.5 or 9.7 inch iPad Pro models.
For the 12.9 inch screen iPad Pro models, if your cassette deck has standard RCA audio stereo outputs, you'll will probably need to invest in an USB audio input/preamp device with the needed inputs and/or other needed audio adapterscable to interface with the audio preamp and cassette deck, as well as, the Apple USB-C to Lightning adapter.
Do a search, either online or on Amazon, for audio interface boxes that are capable of working with iPads/iPhones for getting quality audio into your iPad.
In the iOS App Store, look for audio recording apps.
Most of the better digital audio recording apps will be paid apps.
Good Luck to You!