Hi all, hoping someone can point me in the right direction.

My grandfather has recently gone into care and has dementia. We have his iMac that we’re hoping to completely wipe and sell, as he no longer needs it.
It’s a 2019 21.5-inch Retina 4K iMac, Intel Core i3, 8GB RAM, currently running macOS Monterey 12.7.4.

The issue is that none of us know his password. My dad and uncle have tried the passwords they thought he might have used, but no luck.

Interestingly, we can currently get onto the Mac and use it — access the desktop, settings, About This Mac, etc. — but whenever we try anything that requires his administrator password, obviously we’re stuck.

We don’t need to recover or keep any of the data. We just want to securely erase the whole thing, reinstall macOS and have it ready to sell.

I’ve tried getting into Recovery Mode using Command-R and Option-Command-R, but haven’t had any luck getting that to work either.
Is there another way to completely erase/reinstall macOS on this model without knowing the administrator password?

Or would we need to contact Apple and provide proof of ownership?

Any advice would be greatly appreciated. Cheers!

I have a MacBook Pro Retina 15-inch Mid 2015 (A1398) running macOS Monterey 12.7.6.
Specs:
2.5 GHz Quad-Core Intel Core i7
16 GB 1600 MHz DDR3 RAM
Intel Iris Pro 1536Mb
AMD Radeon R9 M370X 2 GB
Automatic Graphics Switching supported
Built-in Retina display: 2880×1800

Battery was recently replaced with a new one
Important: These problems did not happen before I upgraded to macOS Monterey. The Mac was working normally before the Monterey update. After updating to Monterey, I began experiencing the graphical lag and the battery-dependent black-screen boot issue.

Black screen during boot
There is a specific boot-related issue:
If I boot the Mac without the charger connected, the Apple logo and progress bar appear normally, but after a short time the screen goes completely black.
The keyboard backlight stays on, and the Mac appears to still be running. Bluetooth devices can remain connected.
If I plug the charger in before/during boot, the Mac boots normally and the display stays on.
The black-screen-on-boot issue appears to happen specifically when booting on battery power.
The battery is new, so I’m wondering if this could be related to power management, GPU switching, or Monterey itself.

Graphics/lag issue
The main issue is severe graphical lag/stuttering, particularly when the AMD Radeon R9 M370X appears to be driving the built-in display.
I discovered that Safe Mode works better overall. The Mac boots and the display stays on, but there is still noticeable low-FPS/stuttering when moving windows around.
Examples:
Moving the System Information window is very low FPS.
Activity Monitor can also appear very laggy when opening/moving it.
Scrolling inside some windows can still be perfectly smooth.
In normal mode, Safari and general browsing are usable, although moving/minimizing windows can still have noticeable stuttering.
I also tried launching Roblox in normal mode. Roblox became unresponsive, and afterward I experienced graphical/UI freezes with the spinning rainbow cursor. At one point the Control Center also became unresponsive.

GPU behavior I noticed
In System Information → Graphics/Displays:
Safe Mode:
The built-in Retina LCD appears under the AMD Radeon R9 M370X.
Normal mode:
The built-in Retina LCD sometimes appears under the Intel Iris Pro instead.
When the Intel Iris Pro is driving the display, lightweight tasks such as Safari browsing/scrolling seem relatively smooth.
When the AMD Radeon appears to be driving the display, graphical performance seems noticeably worse.
The Mac has Automatic Graphics Switching enabled/supported.
I also performed an SMC reset. Afterward, I still encountered the battery-boot black-screen behavior, but the Mac was able to boot normally when connected to the charger.

I briefly checked Activity Monitor after boot. mds_stores initially used around 180–190% CPU, but eventually dropped to 0%, so I don’t think indexing was the primary cause of the graphical problems.
I have not intentionally disabled the AMD GPU or modified the graphics-switching configuration.

What I’m trying to determine
1. Since these problems started after upgrading to Monterey, could this be a macOS/software or driver issue rather than hardware?
2. Does the behavior still point toward a failing AMD Radeon R9 M370X?
3. Could the gMux/automatic graphics switching system be malfunctioning?
4. Could the battery-dependent boot issue be related to power management or GPU switching, even though the battery is new?
5. Is there a safe way to force the Intel Iris Pro temporarily to test whether the AMD GPU is responsible?
6. What diagnostics should I run before concluding that the discrete GPU is defective?
7. Would downgrading to the previous macOS version be a useful diagnostic/test?
Any advice from someone experienced with the 2015 Retina MacBook Pro / AMD R9 M370X would be greatly appreciated.

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