This is why I disable background data for individual apps and keep data saver on whenever possible.

Also, browsing can be something like: - Ad blocking - Javascript disabled by default - remote fonts disabled - media eg images disabled if possible - cookies disabled by default

The first 2 are possible using UBlock Origin + Firefox on Android devices. Ad blocking is a must.

Also, you can use some local VPN app like netguard to restrict app's data usage. I don't remember whether you can specify data usage limits though. But it can prevent entire apps from accessing internet.

Then about web pages. On desktop I routinely use text browsers or simple GUI browsers because they are so fast. Even your regular browser can go a long way with JS disabled by default and ad blocking.