I suppose the first question is whether you had the baud rate set correctly. The photo of the “cleaned up signals” (not entirely sure what you did, compared to the prior photo) seems to show a baud rate of 38400, given that each bit seems to take about 15 microseconds.

As for the voltage levels, the same photo seems to show 5v TTL. So it doesn’t seem like you would need a level converter from 15v RS-232 levels. This is one of the few times where the distinction between a “serial port” and an RS-233 port makes a difference, but a lot of data center switches will deal using 5v TTL, because the signals aren’t having to travel more than maybe 5 meters