The Google Pixel 3a camera, unlike the camera in my older Google Pixel XL, takes spectacularly good images through a widefield 5X eyepiece on the stereo zoom microscope:
That’s hand-holding the phone against the eyepiece while manipulating it with the other hand. Definitely not the most stable arrangement, but the camera copes well with slight motions. I really need a gripping hand for the camera, to free up another for the microscope’s focus knob.
For the record:
- Pilot V5RT Extra Fine 0.5 mm cartridge
- Cheap Chinese 1.0 mm pen cartridge
- 0.7 mm mechanical pencil lead
Zooming in (because it’s a stereo zoom microscope and I can), the 1.0 mm ball seems surprisingly un-wetted by its ink:
The Pilot V5 ball seems more smoothly covered:
Those are at the same magnification & crop size, so they’re to the same scale.
This definitely calls for a customized phone-to-eyepiece holder!
The Smell of Molten Projects in the Morning 
Since I already had a Scopetronix camera adapter set for telescopes, it made sense to get the additional adaptors for my microscopes. In your case, I’m guessing (more) 3D printing of adapters is in your future.
This will be the fourth camera peering through this scope. It’d be simpler with an SLR-like camera body and a trinocular scope, but … then I’m trapped on another wheel of incarnation.
If you are looking to steady the phone: The dollar store has a spring-loaded cell phone holder that attaches to a car’s air vent. You can modify the back to attach it to a boom, jib, crane, or other bird of your choosing.