After paying Trump millions, yeah
Yeah those are the main reasons. It's also proprietary, and the only way you to create an account/back up your stuff is by entering a phone number, which isn't exactly great for privacy.
With out bloatware and proprietary Google stuff though
Those conspiracy theories often come up in discussions here on Lemmy, but the TLDR is: Google is a tiny player in the smartphone market, compared to vendors like Apple, Samsung, Huawei, Xiaomi, and others (https://www.statista.com/chart/25463/popularity-of-google-smartphones/). They also serve a much smaller geographical region than most other manufacturers. The Pixel 9 lineup, for example, is only sold in 32 countries. Most of those are wealthy industrial nations. Google doesn't even try to assume market share in developing countries in Africa and Asia. It can also be assumed that over 97% of Google Pixel users keep the Stock Pixel OS, where Google doesn't need a hardware backdoor since they can just implement it in software. So that leaves only a tiny fraction of all users: people in some wealthy industrial nation who specifically buy a Pixel to install a custom ROM. GrapheneOS for example has about 300K users. Do you really think Google would put in the effort to create a hardware backdoor and take all the risk associated with it (negative PR, loss of sales, etc.) just to collect some data about this tiny amount of users? Google already controls EVERY Android phone on the market by forcing vendors to include Google Play Services as a system application through their contracts, licensing and monopolistic market position. Be realistic for a second, and you will realize that your backdoor theories make absolutely no sense and that no business in the world would ever take such a huge risk with such little reward.
Authy sucks anyway.
Not with GrapheneOS, since you can entirely disable the USB controller from the settings on a driver level, making it impossible to connect the phone to a forensic data extraction device. GrapheneOS also has a convenient auto-reboot feature, which (together with their patches to the Linux kernel and Fastboot recovery OS to include memory zeroing) erases the encryption keys from memory, putting the device in BFU state and requiring the PIN/password to unlock. This is additionally secured by the Titan M2 secure element, which makes use of the Weaver API and drastically throttles brute-force unlock attempts. https://grapheneos.org/faq#encryption
All of these are insecure as hell. Linux phones especially https://madaidans-insecurities.github.io/linux-phones.html
Fairphone also really fucked up: They signed their own OS with the publicly available (!) AOSP test signing keys. These guys really don't know that they're doing, and I would trust their hardware or software whatsoever. And no, installing a custom ROM doesn't solve this. Considering how bad their security practices are, we genuinely have to assume that there are security issues with the device firmware as well.
/e/OS is based on the already insecure LineageOS, and it weakens the security further, so it's not a good option either.
None of the options you mentioned can be compared to GrapheneOS. It's currently the best option if you value your privacy and security. You don't have to give Google money either, since you can just buy a used device, which is also cheaper and more environmentally friendly. Google also makes repairing their devices pretty easy for consumers and even works with iFixit. Here's a Mastodon post I recently saw about that: https://social.linux.pizza/@midtsveen/113630773097519792
although you need another trusted graphene phone to use it
No, Auditor can be installed on any Android phone. It's even available on the Play Store: https://play.google.com/store/apps/details?id=app.attestation.auditor.play
You can even perform a remote verification, which uses GrapheneOS servers and doesn't require a second device at all: https://attestation.app/tutorial#scheduled-remote-verification
Security-wise you're better off using whatever OS comes with your device (as long as it gets updates) than downgrading to LineageOS. At least most smartphone vendors (except for Fairphone) manage to ship their Stock OS with a locked bootloader and somewhat working Verified Boot.