Nintendo only bans for cheating in online games or installing pirated software. Nintendo has (so far) not issued any bans for just installing or using custom firmware.

There is no circumstance in which Nintendo remotely bricks your console.

When you get banned for cheating, you are banned from that specific game's servers.

When you get banned for installing pirated software, you get banned from all Nintendo Switch online services. The console can continue to be used fully offline.