Another option, if you want added versatility are the handheld units like my Oregon 450T which I've used walking, cycling, driving (in Furrn parts)
The thing which you will probably have gathered from all these comments is that there is a bit of a learning curve getting to use it to its best effect, no matter which unit you get. Sure, you can use them to good effect out the box, but their usefulness increases the more effort you put into to getting to know it.
As far as routing is concerned, here's a radical thought: you are on a bicycle. You can choose which roads you ride on, you don't have to ride on the M1 EVEN if the GPS tells you to. Trufact.
With a flexible unit (I can only speak for my Oregon but many others will do this) you can set the endpoint then zoom to see the likely route, zoom into the map and then just ride. If you really want to, you can set waypoints at places: towns, road junctions, POIs and just route to those. If you come across a road you don't want to ride..... you just don't.
You've got a route you want to detour somewhere else, just do it. You want to get back to your route, look at the map, drop a waypoint and head for it.
If you expect to have a bike GPS work like a car GPS but for bikes, you will be sore disappointed. It stands to reason that unless the quality of each road or track is ridden and assessed (and never subsequently changes) it will be impossible for routing GPS to work properly for bikes, get over it. Let's face it, most car GPS won't work well for HGVs. If you expect your bike GPS to provide flexibility to keep you rolloing with a smile on your face knowing you always know where you are, it will do that.