For my commuting use, I'd be happy with a D4S, but preferred the D11 as it's a bit lighter and cost wasn't an issue for me. It's been great for that.
For your use (Canal riding), I'd buy the RadRhino as you were thinking, except I wouldn't want to be manhandling 33kg at age 70 up and down steps and ramps and other obstructions you often find along canals (I'm basing this on a 30-mile section of the Grand Union I cycled), so maybe something similar but lighter to handle? I have two other larger wheel e-bikes for leisure, and all my bikes have been along rough tracks and canal paths. The Fiido is 17.5kg and light to swing around, but small wheels and absent suspension is a bit of a trial/hard going with prolonged use on rough/pot-holed/unsurfaced canal & railway & field tracks, but can be done. My other e-bikes are full-size hybrid framed and weigh circa 20-22kg: They handle better with bigger 700c wheels and front suspension for that style of outing, and are more easily carried up/down steps and stairs, which you often need to get onto, off, and when crossing over canal towpaths.