If you never enable any data services on your SIM (GSM only for calls) and never connect to WiFi. You're not leaking anymore data than an old fashioned phone.

However, the fact that you are asking these questions does suggest you are probably being unhealthily paranoid. (Reading these replies, it seems you are in good company on this forum.)

I'm going to assume you are not engaging in high level cyber activities that would require you to adopt this level or paranoia. If you were, you wouldn't be asking basic questions.

Instead, it sounds like you have developed an irrational fear of what tech companies can realistically do with your data and what level of harm they may wish to cause you.

Typically, they can log your search and browser data to determine if, for example, you like pizza. Then they may show you an advert for pizza or highlight the nearest Dominos on Google maps. But... they can only do that if you share that information with them in the first place by using your new smart phone (with none of the privacy settings enabled) to search for pizza and then using Google maps. Nobody is forcing you to do that. But is it really that bad even if you do?

Google are not going to clone you, or assassinate you or somehow work out you are not paying taxes or are engaging in illegal activity unless you use your phone to do it. And even then, they don't go round grassing people up to the government for the fun of it. They just want you to click on adverts and, once you are aware of how they operate, it's relatively easy to avoid them whilst still getting great value from a pretty incredible piece of modern technology.

Now, if you are genuinely worried about government targeting (I don't know what country you live in) an encrypted messaging App will be significantly more secure than an unencrypted old GSM phone that is quite easy to intercept and relies on the integrity of your MNO provider.

My advice, stop worrying. You already have a cautious mindset so you won't get tricked by these companies, but you could also be enjoying many of the benefits of being able to access all of humanities collective information from your pocket.