I'm also in this situation where I have same kind of old iPad, which was lying in a drawer at my parents. It's running iOS 9.3.6, for which only a semi-tethered jailbreak exists. IIUC, this means that you can jailbreak it by side loading an app to the iPad.
The good thing is that you can do all this jailbreaking with free software from Linux, apparently. Of course it's endless browsing shadier and shadier / scammy websites before finding something that looks somewhat less shady. The world of iOS jailnreaking is very strange.
The bad thing is that sideloading requires an Apple account. I don't have one. Creating an Apple account from the device is not possible anymore. Creating an apple account from the apple website seems complicated. It wants a phone number which I'm not willing to give. I tried these services that provide burner public phone numbers to get the verification code it wants but have yet to find one that works. When i manage to get a verification code, it tells me that this is not possible at this time, please try from an Apple device. I don't have another one. I suppose I could try to run macOS in a virtual machine just for this.
I'd like to make this device useful. Up to recently I could use it to display music scores in PDF, I even wrote a web app for it to annotate them, but the screen is a bit broken and this makes the touchscreen very erratic to the point of being unusable.
I'd be happy to make it useful for something else but on the other end, I have other things more interesting to do than to deal with Apple's bullshit.
Too bad.