I agree that the 4a is simply awesome. It is too slow for GrapheneOS, but the size, the way better fingerprint sensor that just works, and even the cameras are way better than a 6a for whatever reason.
But this is because Google makes stupid decisions. They mess up UIs, and they follow the same moneygrabbing trends of glass backs, bluetooth headphones and huge sizes as everyone else.
GrapheneOS cannot maintain their own kernel, and even then they have literally no way of supplying Firmware patches, as only the manufacturers can update the firmware. I would say it so: Android is so secure that a big part of vulnerabilities comes from Firmware.
It is not possible to support an unsupported phone, I am sorry. I use my 4a as an alarm clock, offline navigation and dictionary and a few more things. I think connecting to trusted services only from home, and on the go shutting off every antenna makes it very secure too.
And sorry but NTFS support is barely on Linux. Just because Windows uses that filesystem no other OS is obligated to implement it.