To be honest, this is all about threat modelling. If you have it on airplane mode all the time, it is already incredibly more safe.
Then if you use it offline (and there are tons of things you can do with a mostly offline phone, navigation, dictionary, music, books, movies, ...) it is almosr completely fine.
All depends on what you actually do with it and what your threats are.
Try DivestOS.org
They target EOL devices to reduce harm. It is very close to GrapheneOS, while also a big portion less secure. But for your phone it should be perfect. They do things like port latest Android, which improves security a lot.
https://divestos.eeyo.re/pages/devices