i disagree about slow and stable as possible. I've flown slow flyer type planes that were challenging to fly. When the plane's top speed is only double the speed of the wind you're flying it in, things get uncomfortable in a hurry. And yes, too stable is certainly a thing. A plane that fights your control inputs won't save you from bad piloting and certainly won't advance your learning.

A much better strategy is to choose a plane with a slow stall speed but a significant flight envelope that has some self-righting characteristics while still being reasonably responsive. The old Telemasters, Kadets/LT40s, and other classic trainers all fit that description when trimmed and balanced properly. The venerable Ugly Stick does too, but without the self-righting traits of a traditional trainer.