I'd go to the Garmin Forums website
https://forums.garmin.com/index.php
and start posting there. the deal is that of course in those forums, you get explicit feedback

for what its worth...
I use a Garmin 705, and pretty much have had the same type of issues.

that is to actually use the GPS more than just a cyclometer

routing... I use both US Topo and CN North America
the routing function is ok...
typically I use my iMac, using BaseCamp, and make tracks for whatever my routes on tour are going to be

on my 705 I have CN North America
then I pick and download the specific sheets for Topo
I'm using a 2GB card