I agree 100% with you Bob. The same thing happened to me. For example when I started "playing" (more or less ...) keyboard and piano, years ago. Or, more recently, when I tried the banjolele and the mandolin. A new instrument always brings out new ideas, as you said so well. And complex instruments also work, in that respect.
But I was really surprised about what a really, really simple instrument like a uke, probably the simplest of stringed instruments, did for my creativity. I was amazed. And I think that 4 is the key number here. Having 4 strings allow you to play a wide range of chords, but not too wide (jazz players will disagree with me, I'm sure .:-). 3 strings would be too limiting, I think. And 5 and 6, it would be too "tempting". 4 seems to be just right ... to me. But I still love guitar of course, I still write songs on guitar, it's still "my baby" :-)