One day, I decided to downgrade my firmware and after that, I put a game cartridge in and I totally killed it.
What exactly do you mean by "You totally killed it?"One day, I decided to downgrade my firmware and after that, I put a game cartridge in and I totally killed it.
My switch can't even send payloads at this point.In all likelihood you didn't "kill it." The cartridge reader has it's own firmware which sometimes is updated when the system firmware is. Depending on the version(s) involved, you may have downgraded your system's firmware to the point where it is no longer compatible with the newer firmware running on your cartridge reader. But usually everything will still run fine as long as you don't have a cartridge inserted, or you can disable the cartridge reader altogether by setting nogc = 1 in stratosphere.ini (assuming Atmosphere.)
Or, you can update your firmware back to whatever it was in order to fix the cartridge slot.
Yes but we are asking what exactly you did. Be very detailed.My switch can't even send payloads at this point.

How do you know that? Have you verified your console is in RCM?My switch can't even send payloads at this point.