Therefore, from a security perspective, the phones with the best security are the iPhones and the Google Pixel phones. They also generally have the quickest security updates.
If you install a custom ROM (e.g. LineageOS), you can make almost any phone reasonably secure, but that involves a lot of time and hassle. If you don't trust Apple/Google, that's probably your best option.
An alternative approach is to have a second device for security sensitive purposes, which can be locked-down, only essential apps installed, offline by default, etc.
Do the official LineageOS releases have Verified Boot (allow you to lock the bootloader)? Everything I've tried uses userdebug keys, which is pretty bad security-wise, and requires an unlocked bootloader.
I ask because Verified Boot ensures the root of trust is there in the binaries. If they're wrong by one bit, the OS won't boot.
Realistically, even if your OS has additionally security features compared to AOSP, it'd still be like building a house out of obsidian, then putting a wooden barn door on it.
Possible in theory, but complicated:
These attacks require physical access to the device, and a rather sophisticated adversary.
If the attacker has physical access to the device, they could plant a camera to get the passwords or use a $5 wrench, etc.
At that security level, you can't trust a single device, and you'll probably need to look at security solutions such as hardware security keys (Yubikeys), Shamir's Secret Sharing, plausibly deniable encryption, etc.
Didn't Librem try to work around this? I think they sealed it before transport, and used nail polish to ensure the device had not been tampered with in transit.