As far as I know, getting a newer phone is your only option. It doesn't have to be a brand new phone. Andriod phones newer than 2015 should work. The OS requirement is Android 6.0 or IPhone 13.

You could ‘borrow’ a phone for the duration of your trip, or use the paper pass, instead.

Best to upgrade your phone, though. New smart phones can be bought cheaply, these days.
A nice new Nokia C21 Plus is just £100 on Amazon right now.
You can easily switch the app to a new phone. Your pass will only be registered on ONE device, so if you try to register the pass on a new device it will ask you if you want to ‘disable’ the previous pass.
I was hoping that I could register the app on both my phone and tablet and that they would ‘sync’ across devices. My intention was to plan my journey on the nice big tablet, but keep all the documents for travel on my phone, but… no go with that!

The app is anyway not good for planning so you better do your planning on your tablet using the timetables of the national railways and excel. Then, shortly before your travel you then add your trains to your Trip in the app. Doing like this you hopefully avoid the problem with not finding your train in the app due to the app not being updated that frequently.