The google phones have always been great with not having bloatware, if you don't count the pre-installed google apps. In the past I've had Nexus phones and they were great.

GrapheneOS only works on Pixel phones, but it was very easy to do. They have a web installer and good instructions. You plug in the phone, wait for Windows to install drivers (I gave up on Linux 😆), then run the web installer. There are a couple of steps you need to do on the phone in regards to unlocking and re-locking the bootloader, but the installer takes it to the right place and you just have to push some buttons to confirm.

Overall it took maybe 20 or 30 mins but was very easy. I'm not sure if it's the same for other OSs or if it's particularly easy because it only works on very specific devices so they can control the process a lot more.

Though if I was planning on installing an alternative OS, I'd make sure my phone was on the supported phones list before buying it. Don't want to brick a new phone! There are several different OSs that support a wider range of phones than GrapheneOS so hopefully you can find something.

My understanding though is that GrapheneOS (or maybe it's the pixel phones) is the only one that lets you re-lock the bootloader, and from a security perspective leaving the bootloader unlocked can be an issue (that's physical security, where someone steals or has access to your phone, not general internet risk).