How do I free up the serial port on the carambola 2 module so that I can use it for my application?

I followed the directions here: https://forum.openwrt.org/viewtopic.php?id=41826

This works almost well enough. Two minor issues:

Some initial boot messages are shown, but I can use GPIO to enable the serial out of my board. So this is not a big problem.

Character echoing remains on the console. Verified with a scope. This character echoing remains until something else accesses ttyATH0. What I did as a test was ssh into the carambola 2 and run "screen ttyATH0 115200". I was able to send characters between my ssh session and the console port as expected. Once I terminated screen character echoing was no longer active on the serial console. Is there a way to disable this character echoing on startup?

The ideal solution for me would be to have the serial console enabled by a gpio line on startup. If for example and GPIO is high then the console works as normal, but when low the console is completely silent. Is this possible?