/e/OS is not fully degoogled, as DNS connectivity checks, hardware attestation provisioning, and eSIM activation all go through Google.
It is often many weeks or months behind on security updates, especially in the WebView, which makes it easy to exploit.
It doesn’t support bootloader locking on many devices, and if you lock the bootloader on a phone that does support it, it could brick if /e/OS is on an older security patch than the stock ROM was.
It doesn’t use a lot of the hardening in GrapheneOS such as hardened_malloc which prevents memory corruption exploits, even if the hardware supports it.
And finally, /e/OS’s text-to-speech sends what you say to OpenAI, despite local options being available.
If you want a properly secure Android phone, the best option is GrapheneOS, however it only supports Pixel phones and future Motarola phones due to its high security requirements.
If you can’t get a Pixel then iOS in lockdown mode is the next best option, however if you can’t replace your phone, LineageOS is much worse than Graphene although it is still much better than /e/.
Smart phones are a bad idea. A simple, dumb phone to make calls, texts and occasionally tether your laptop, vehicle tablet to for data access are all you really need. Even the dumb phone should have physical switches for the radios and a battery that can be removed without any tools.
The problem is that real dumb phones are hard to find. Many modern “dumb phones” are actually full android devices, complete with a boatload of spyware that helps keep the cost of the device itself low.
KaiOS is better but that’s a whole linux distro, with similar issues.
Since you mentioned tethering, do you have an example of a non android (or at least one that’s not preloaded with a ton of spyware) dumbphone that supports usb tethering? I am skeptical that a real dumbphone would have this feature.
Nokia 2780 is not bad. It runs KaiOS, supports tethering and 4G cell data. As long as you stick to the script on how you use the device it will have minimal privacy issues IMHO. If that is still not enough you can skip phones altogether and get a 4G dongle for your favorite Linux laptop/tablet and just use a softphone + voip service.