Yes, sim calls and sms are not private, both on dumb and smart phones. They also connect to cell towers so your location might be found. Anyway, I think the hardware and software of a smartphone is more capable of surveillance you than a dumbphone. The only realistic way is to leave the phone home and keep a paper list of your contacts at you. And call them from a stranger if you need to. I did this and the biggest inconvenience is that I can’t take pictures or videos (I might buy a camera, I don’t know if they have location system incorporated).
Great decision to move to GNU/Linux. I used Mint and recommend it as first distro for a Windows begginer. I always use a USB stick for back-up. Enjoy the journey!
Great news. Public institutions should never buy or use proprietary software.
There is a great cheat sheet made by Ubuntu.
I have it and used it in the past. It’s better in privacy than Signal since it’s decentralized and doesn’t use any identifiers. Signal forces you to use the worst identifier ever: your phone number. It should be optional (as Threema) for easy contact discovery, not mandatory. Back to SimpleX, I stopped using it because every time you have to connect the computer to phone if you want to use it from computer (it’s for maximum security, but inconvenient for me). But it’s a great app.