When comparing against something like a Pixel running GrapheneOS, it's honestly a bit more puzzling to me. Granted, I'm definitely not the audience for this, but with G_OS you can do most things that a regular phone can do, without taking several minutes to install Firefox.
As much as I love privacy (going as far as having a semi-random username), this phone is a bit puzzling. I hope someone can throw more light on this.
I'm not familiar with GrapheneOS but I assume it follows the usual model when repurposing Android devices of taking various closed source blobs (i.e. drivers etc) and rebuilding the open source bits around them? If so, this approach usually locks you into a Linux kernel version to remain compatible with the blobs which limits you on kernel features and fixes as well as who knows what exposure the blobs have to offer, which also will likely never get updates.
I would disagree with 'has to be comfortable to use' as that is often at odds with 'secure'. Some of the things I do to secure my system make infrequently done/high risk things quite uncomfortable to use. Not because I wanted to make it uncomfortable, but because that's what it took to get the level of security I desired.
[1] I would also argue that spending too much effort here before addressing other attack vectors first is rather silly. (i.e. web browser, network, minimizing usage of/isolating 3rd party binaries)
I also have to agree with you on the “one can make it secure” part, e.g. android builds on top of pretty standard linux tools to achieve its better security, namely selinux for a larger boundary and the most important: running different processes as different users! It is such a gaping security issue in typical DEs, as otherwise not even the very crude UNIX permission system can do anything meaningful (other than the relevant xckcd comic: the attacker can access all my files, my browser cache, etc, but at least can’t install a video driver)