Also, system-wide adblocking either has to be done off-device (no good for cellular networks) or with an on-device VPN, which is no good if you actually want to use a VPN of your own.
Personally, I stopped rooting a few years ago and I don't see any ads with Firefox + uBlock Origin; I just don't use the type of apps that would show ads; not necessarily intentionally any more, just that the types of services I install apps for don't do it (making obvious exceptions for things like Amazon where the entire app is an Ad).
Sure, but that's "off-device", so you either have to run it yourself (and pay-for-and-manage it) or rely on someone else to run one for you, which if you're into privacy on top of ad-blocking, you likely don't want.
Yep that's fair. I was thinking you had to have root to install any modified apks but I could very well be wrong there, its been a while since I got that deep into modifying Android.