Idk if this app is on Apple or not, but it's called ReadEra in the Google Play Store (https://play.google.com/store/apps/details?id=org.readera). If you can find this app, or another reader like it that reads Epub documents, install that, then go to the ao3 site to the story you want, and there should be a download option for the story at the top of the page. Click it, choose epub (or any of the options that you'd be able to use on the reader app you picked), and once it's downloaded, bring up the reader app and have it scan for downloads.
ReadEra is super simple because you just download the story, scan for new downloads, and then it's right there in the library and you just start reading. And it has no ads that I've come across since using it in the last 4 months give or take. Layout is super easy to use. And if you like TTS function where it'll read the story aloud to you the way FFN app does, you can get Evie (https://play.google.com/store/apps/details?id=com.lenntt.evoicereader), or something similar to it if they don't have that on Apple, and the process is the same. Download the story, open app, scan for downloads, read.
The difference there is that you may need to have TTS files to begin with in order to use it. I've always used Android, and they come with Google Voice TTS files preinstalled on every device, so I'm not sure how that works for iPhones, sorry. But I hope this helps!
I keep an Airtable database of over 700 recommended fics, which you can sort and search by character, ship, rating, or author. Most of them feature Minerva McGonagall because that's my OTC, but you can find others mixed in as well.
I've written a bunch of Minerva-centric stories you can find under my username on AO3, and I curate a recs list on Airtable that allows you to search for stories by character and/or pairing. It contains something like 600 pieces of fic and art featuring Minerva.