I do a lot of sonar logging to get contour mapping so I have some suggestions for you. I use Auto Chart Live, Sonar Charts Live, and Sonar Charts.
As you navigate, your unit draws a Track on the Chart view. That Track is made up of Track Points. Every Track Point has the depth and location. Those Track Points are created every second. When you are recording sonar logs or doing the Auto Chart live, those Track Points are what positions the contour lines.
With your unit on the bow with the transducer on the trolling motor, your sonar data and position data are at the same place. That makes your mapping accurate. If you put the unit on the console and transducer on the transom without adding an external GPS antenna at the transom, your map will be inaccurate by the distance between the GPS antenna and transducer. Additionally you should set the depth offset so the depth is recorded instead of the distance between the transducer and bottom, if you don't, your map will be inaccurate.
You seem concerned with the speed of doing mapping, think about this: the unit creates a Track Point every second, how far do you travel between Track Points at the speed you want to go? If the depth changes in a short distance, will there be data to indicate that at the speed you want to go?
I did a 420 acre lake that is electric-only using a bow unit to record the data for the Navionics Sonar Charts program. It takes time to get accurate contour mapping and the ONLY accurate part of any contour mapping is what you pass directly over. The mapping software will create contour lines much wider than what you record, all programs do that. You have to overlap those lines to blend them so most if not all of the contours are real and not fake (interpolated).
Examples:
This is the 420 acre electric-only lake I did with sonar logs processed by Navionics Sonar Charts program:
This is the same map with Sonar Charts Live contours overlayed on it. The wide line at the top is a single track created by the software program. The darker area at the bottom is blending several Sonar Charts Live tracks:
This is same lake with some Auto Charts Live tracks overlayed on it: