First off, just avoid Google Pixel devices entirely, NO exceptions, this is for obvious reasons and being basically the quintessential anti-privacy company (Aside from Facebook maybe).

What you listed has no bearing on how good or bad Pixels are. Besides, Pixels are a minor income for Google and not purchasing one isn't going to cripple them.

They also run proprietary firmware

All phone firmware is effectively proprietary, because it is exceedingly difficult to verify the high level code especially on proprietary architectures like ARM.

that has integrated tracking, do NOT trust it

False.

I would also completely avoid iPhones or any Apple products, due to their operating system being completely proprietary which means that

Being proprietary means almost nothing in the context of a piece of software itself.

A) Their security is awful

False, the FBI could not get into the San Bernadino shooter's iPhone. iOS contains strong sandboxing and exploit mitigations, among other things. Besides, being proprietary does not reduce security as I will mention later.

B) they could track you whenever they want to without you knowing.

iOS does not do this. Perhaps Apple services like iCloud do, but not iOS.

My personal recommendation is Xiaomi phones

Xiaomi is a Chinese corporation, meaning it is a CCP shill/branch/slave. It's unethical to support it and also self-defeating since the CCP does shady things.

which mostly support LineageOS, currently the most secure and private usable custom ROM

LineageOS is extremely insecure, such as disabling verified boot: madaidans-insecurities.github.io/android.html#lineageos and without security you cannot sustain privacy.

They are also quite easy to root to improve your security and minimize tracking.

Rooting substantially reduces security. AOSP and LineageOS don't have telemetry so rooting doesn't "minimize tracking".

I have also heard good things about samsung's phones and ASUS' phones in terms of security and privacy, especially with MUCH better track records than Google.

Samsung phones are not especially secure, especially with custom ROMs. Asus phones do not seem much better. Google phones are not privacy-invasive and are excellent for security.

But overall Linux Phones are by far your best option for privacy and security.

This is completely wrong. Linux phones are very insecure as they pretty much port desktop Linux, already very insecure, to phones. madaidans-insecurities.github.io/linux-phones.html

I highly recommend LineageOS, which is currently the most secure custom ROM you can use right now with minimal sacrifices

This is false as I have already explained.

and I am also personal friends with one of the head developers of it, and I know they are good guys with good intentions

This changes nothing about LineageOS itself.

GrapheneOS is nearly unusable for regular people due to its lack of speed

False.

and Google services, which breaks many apps

Most apps that require Google Play Services often have an alternative that does not, or have a web app that does the same thing. In any case, the project will implement a compatibility layer, implementing a choice of microG, a spoofer that pretends services aren't available, or nothing. If you still need Google Play Services then you can easily use CalyxOS, which is only slightly less secure than AOSP and much more secure than LineageOS.

as well as mainly being for just Pixel devices

This is a non-issue as mentioned beforehand.

But most of all: AVOID ANYTHING PROPRIETARY AS MUCH AS YOU CAN!

There is no reason to avoid software simply for being proprietary.

Proprietary software can track you without anyone knowing

It is trivial to monitor your network using Wireshark or router settings. If it doesn't exfiltrate data then it's a non-issue.

Proprietary software can do anything and you would never know what it was truly doing.

This is false. Proprietary software is not some black box which you can never understand. It can easily be analyzed and often is; this is why you can find vulnerabilities in proprietary software.

Also, if you couldn't analyze proprietary software, then it would be more secure and Windows would never have any malware because no one could ever find vulnerabilities. This is wrong however. The source model matters very little regarding security, and at most it's that once a vulnerability has been found anyone can submit a patch for open-source software. This is maybe a 5% help. This is why

I would also completely avoid iPhones or any Apple products, due to their operating system being completely proprietary which means that A) Their security is awful

is wrong.

Your guide is misinformation. Probably unintentional misinformation and you made an honest mistake, but misinformation nonetheless.