I'd start with a rich, full-ish sound on the upper, with a mellow combination on the lower and pedals. If you want to learn how to play 'home organ', then I wouldn't use the upper manual that way. 1) does the sound combination you use sound good to you and accompany that piano type sound, and 2) you need to use sounds that run through a tremolo or tremulant that does not affect the voice you're using to create the piano effect. As the organ can only make organ sounds, apart from the synth on the solo manual, any combination of stops will make the kind of sound you want.

If possible, you'd use something like a trombone or diapason and either add sustain or some type of percussive attack. You'll have to fake it as we did back in the late 60s and early 70s. If it's a 4373, there won't be a piano sound as such for the upper manual (what you called the 'middle').