You got the letter because you used torrents and it is considered sharing/uploading copyrighted material. Some companies place clickbait torrents in purpose and monitor who is going to start downloading. Then they send a letter with scary text to the ISP and wait for you to reply. The ISP has to forward the letter to you, but they don't reveal who you are. Court order is needed for this. There is no court order because the letters are send about thousands of IPs, not people. When you reply directly, you reveal your identity yourself and turn from an IP into a person, subject to further attacks and threats. This is how this scam works. Nothing new and nothing too dangerous. Just don't reply.
You don't need to run all your network through VPN all the time. Run it when needed only, if you feel more comfortable this way. Streaming is not sharing and is not illegal in many countries. It is illegal for the source, but not for you. Your Firestick users are safe. You fell victim of common scam and spend monthly extra money to "protect" yourself with no real need to do so. I'm assuming you are talking about family members. As I said, your protective actions hurt everyone's Internet experience all the time, needed or not. Why do you pay for Gigabit Internet service?