I mean, exactly how invasive are default operating systems? (Like Windows, Mac, Chrome OS, Android, iOS) Do they log your keystrokes, log passwords, capture screen, upload your photos, videos, or audio? (Assuming you aren’t a target of government) Is it even possible for the average person who doesn’t feel comfortable messing with installing operating systems to have any privacy?
We don’t know if they do, but they certainly can. Especially if you are on x86. I’m sure Android (which comes from OEMs) and iOS devices spy on you.
No, you have no respite unless you switch to custom. The good part is that this process is much easier than before (especially on the desktop), and will keep getting easier. Graphene already has their Web installer when you plug your mobile into your computer and let it do its job. Installing Linux is the easiest it has ever been, and I would argue that this trend has creeped into even the more advanced distributions like Gentoo/Funtoo (their guide is extremely well written and easy to follow + forums).
The only thing you are losing is time. If you don’t have the time, then no, you should stick with the easier ROMs/Distributions. I would never espouse using Windows/MacOS/OEM Android/iOS unless forced to by circumstance.