LineageOS alone has around 4 million active users. So malicious use is at most 1:4, not 5:1.

Assuming that we accept your premise that the most popular custom firmware for Android is stalkerware (I don’t). This is of course, a firmware level malware, which of course acts as a rootkit and is fully undetectable. How did the coalition against stalkerware, pray tell, manage to detect such an undetectable firmware level rootkit on over 1 million Android devices?

> The entirety of GrapheneOS is about 200K downloads per update. Malicious use therefore is roughly 5-1.

Can you stop this bad faith bullshit please? "Stalkerware" is an app, not an alternate operating system, according to your own source. You're comparing the number of malicious app installs to the number of installs of a single 3rd party Android OS which is rather niche to begin with.

You don't need to install an alternate operating system to stalk someone. And in fact that's nearly impossible to do without the owner noticing because the act of unlocking the bootloader has always wiped the device.

> The Coalition Against Stalkerware defines stalkerware as software, made available directly to individuals, that enables a remote user to monitor the activities on another user’s device without that user’s consent and without explicit, persistent notification to that user in a manner that may facilitate intimate partner surveillance, harassment, abuse, stalking, and/or violence. Note: we do not consider the device user has given consent when apps merely require physical access to the device, unlocking the device, or logging in with the username and password in order to install the app.

> Some people refer to stalkerware as ‘spouseware’ or ‘creepware’, while the term stalkerware is also sometimes used colloquially to refer to any app or program that does or is perceived to invade one’s privacy; we believe a clear and narrow definition is important given stalkerware’s use in situations of intimate partner abuse. We also note that legitimate apps and other kinds of technology can and often do play a role in such situations.

- https://stopstalkerware.org/information-for-media/

This assumes a high level of technical skill and effort on the part of the stalkerware author, and ignores the unlocked bootloader scare screen most devices display.

If someone brought me a device they suspected was compromised and it had an unlocked bootloader and they didn't know what an unlocked bootloader, custom ROM, or root was, I'd assume a high probability the OS is malicious.