I’ve never gone down that route myself, but I have both an iPad 2 and iPad 3.

The iPad 2 is downgraded to iOS 6, making it significantly useful and faster, and this can be done with the Legacy iOS Kit. That being said I did this over a year ago when the App Store still worked on iOS 6, so you’d need to instead find IPAs and install them, which is definitely a more subpar process. I have a metric ton of old games on mine, as well as TwomonUSB so I can use it as a second monitor

My iPad 3 is downgraded to iOS 8, which is maybe a 15% speed improvement but it is more bearable than stock iOS 9, and the app store works as well as more apps like Spotify, Discord (with tweak), Telegram (editing Info.plist). Against the grain of what r/LegacyJailbreak would suggest, downgrading to iOS 8 is probably better than iOS 6.

This doesn’t answer your Linux question, but if you wanted to get more use out of your iPad in the meantime I love having mine as a second monitor for my laptop (could also use VNC instead of TwomonUSB) and in general what I’ve said above makes them 100x more useful than on iOS 9.