ROM means read only memory, you are not modifying the phone’s read only memory. The term originated back operating systems and games were intended to be written once to a chip at the factory and never modified again.

If you have a pixel, I’d highly recommend avoiding custom OSs other than GrapheneOS unless you cannot use GrapheneOS for whatever reason. That said, in most cases running stock with modified settings can preserve the base android security while giving you more privacy.

If you can, you mention having a Samsung, avoid using Samsung with a 10 foot pole. Their security is laughably bad for a major OEM, and once you begin customizing anything with the phone, you’ll encounter bug after bug after bug.

If you want a handful of reasons to avoid non GrapheneOS custom OSs, here are some of the biggest offenders (if you want more, you could ask in various GrapheneOS chatrooms (such as on matrix or discord) and they’ll be happy to give you more reasons (be warned, some people will be very blunt)):

Most other custom OSs use privileged MicroG, as opposed to sandboxed Google Play which is mostly unique to GrapheneOS, as well as their own privileged serviced to provide things like parental controls or deeper app customization.

They’re also very lacking in security, being no better than stock android when faced against Cellebrite or Graykey.

Their updates tend to take weeks, if not multiple months in some cases, while also not updating when new vulnerabilities are found unless it’s patched upstream.

Most don’t support locking the bootloader due to the way they support devices (this is part of the reason GrapheneOS is pixel exclusive) which means your phone is vulnerable to having parts of the boot process modified potentially maliciously to allow an attacker access to your data.

User privacy on custom OSs is often really theater, promising privacy because they’ve removed Google Play Services by replacing it with a less secure, more vulnerable, and less user friendly version called MicroG.

Finally the /e/OS devs, an operating system recommended to you by another reply, straight up said they’re intentionally not making their OS any more secure, because only pedophiles need to worry about their privacy (the video is in French, and I don’t have it saved, but I can find it if you want).

All said, if you just want to avoid Google but don’t care about any other Tom, Dick, and Harry accessing your private information, custom OSs are great. If you care about other people, especially other companies, not having access to your private data, and you’re already getting a new phone anyways, I strongly recommend buying a pixel and slapping GrapheneOS on it. Sideofburritos is a great source of information for GrapheneOS, he even made a video trying to brick GrapheneOS during the install process (spoiler: he couldn’t) in case you’re worried about damaging the pixel in the process.

Disclaimer: I’m not associated with the GrapheneOS project, I’m just a user who cares a lot about their digital security. If it sounds like GrapheneOS is too good to be true, I’d be happy to include some drawbacks of the OS. There really aren’t many, as every news outlet that reviews the OS can agree, but there are a few.

Also check /e/OS and iodéOS, they support some Samsung phones.