I'm a beginner and have a few questions I'd like to ask

I would say everyone should move away from gmail. The big tech firms have all been shown to ban users at the drop of a hat effectively cutting them off from significant parts of their digital lives. That said, I would also argue that you should pay attention to where they provide value and where they extract it. I have a lot of concerns with Google and how they do business, but it’s hard to argue that they don’t offer useful services for “free”, even if it is in the “you don’t pay, with money” sense. I use both Proton and Tuta for different things and I would recommend either. But I would also recommend transitioning yourself as opposed to trying to jump ship. Both have categorisation, but they don’t have smart categorisation. Tuta requires a paid plan for Inbox rules. Proton does appear to allow filters on the free plan, although I assume there’s a hard limit on the number of filters you can add. Gmail does state that they do not use message content for advertising. But I’d argue that is in part a result of the value of metadata analysis, not out of respect for privacy. Sender and subject aren’t traditionally considered part of the message content, even attachment names might be argued as separate. Google has such a large data trove that for example, if you use email to correspond with your doctor, Google might be able to figure out what medical stuff you’re dealing with purely as a result of which doctors are included in email correspondence. They’re analytics is trying to beat blind guessing. They don’t need to be accurate, they just need to beat the odds. Who you talk to and when is worth more than the subject (especially if Google also knows something about them). The subject and correspondent list is worth more than the content, and are less prone to manipulation. Public backlash did probably play some part in changing their policy, but I wouldn’t be surprised if was changed mainly as a result of content driven ad targeting being costly and ineffective in comparison to scanning email headers.