Android

I currently own a S23+ and had a OnePlus 6T before that and Nexus 5X before that. I wouldn't be bothered by the Samsung bloat. You can uninstall nearly everything. Set the default apps you want to use and hide the rest. You have this configured quickly and then you will never be bothered by it again.

The default Pixel 8 launcher doesnt even allow you to remove the huge Google search widget, which bothers me a lot more. I find One UI pretty vanilla feeling to be honest. Don't notice that much difference from my OnePlus 6T which had a fairly vanilla OS.

I have the Pixel 7 Pro and there's no ad forcing. Some apps can push ads, but just block them or don't use those apps. I'm using the Nova Launcher though. Idk if that matters.

They will hate me but iPhone has BSD/XNU/openSTEP. Of course it may feel like a prison after Android. Either that or a Linux phone with Waydroid

I have the Xiaomi 13T Pro, and I haven't noticed any ads. You can buy wallpapers and ringtones, but there are also tons of free ones, and it doesn't pop up advertisements for it except once to show the function is there. The new HyperOS although it looks the same and has similar functionality on the surface as MIUI, is way way better IMO.
Security and game center are gone, 2 of the top annoyances with MIUI IMO. HyperOS feels way snappier too. With MIUI the 13T Pro was slower than my old Motorola to turn pages in an e-book quickly. Now it's super snappy. HyperOS just feels better somehow.
I installed Firefox with uBlock Origin, and chose it as default browser. I also uninstalled "Community Center" I think it was called. An app to use various Xiaomi services.
There are a couple preinstalled apps, I don't recall if Facebook was among them, but they are easy to uninstall, it literally just takes a minute to uninstall.

But maybe Motorola would be more to your liking, they use almost completely vanilla Android, with only an app to configure Motorola specific features. I did get some ads for printing services on my Motorola, but it seemed to be more a Google thing than Motorola. (Haven't seen any such things on my Xiaomi 13T Pro)
And AFAIK you can still remove the search widget from the desktop on Motorola, something that apparently is no longer possible in completely vanilla Android. Reason enough for me to completely avoid Google Pixel phones. Why would they even want to do such a stupid thing?

The easiest way to get rid of ads it's a DNS level AdBlock, you can simply use the private DNS option of Android(with AdGuard or Mullvad free DNS resolvers), next step use alternative frontends for YouTube(NewPipe x SponsorBlock) and Music like Spotify and YT Music(xManager or ReVanced), there will be some apps what will keep showing ads like Facebook but at least it will be a minor amount.

If you want to be more sophisticated about Android AdBlocks I will suggest you to root your phone, but with these non-root options 90% of people will do pretty fine.

I'm in the same boat, the issue is my pixel 6 pro has a big smash on the screen so the Graphene option isn't possible. I used to have a Xiaomi and actually really liked it but I hear they're changing the os/UI? Also I dunno about ads, that sounds crap.

Oneplus

I'm using a pixel 6 non pro, and found that the pixel 7 and 8 were both a downgrade in terms of performance. Sticking with the 6 for now