If I have hbirds at the console for the sidescan, but garmin up front, how are people managing the waypoint translation? Just kind of eyeballing it? I'm especially thinking about when I get close enough to drop the TM, how are you staying on target? Or do I really need to also have an hbird up front for this too? 

either another bird up front or FFS to find the 'thing' you marked on side scan.  I don't know of any way to share across brands.

If you have a HB console unit networked with a networked MK trolling motor, you can use the "go to waypoint" feature and spotlock on it.