Welcome to the forum.
I’ll give you some broad ideas about solving your issues, if you want me to get specific, just reply to this post and tell me what you want to know more about.
If you can’t install a secure and private custom ROM, I would advise you to uninstall as much bloatware on your phone.
For those apps you can’t uninstall, use an ADB command on your computer.
Only leave the applications that are essential for Android to work (trust me there are a small number)
Be careful though when removing apps, start with the obvious ones (like for example your phone doesn’t need to have candy crush preinstalled to function).
After that you can install a DNS level firewall like nextdns.
Is dual booting an option?
What kind of games do you play?