Well, I'm not sure. FFII DoS was quite tough, and Souls of Rebirth was extremely hard for me. FFIVA had its difficulty toned down, but I think it was still harder than FFIV Easy Type.
As for FFI DoS, I quite agree with you. Even though it's easy, the gameplay is smooth and enjoyable. Walking through dungeons and towns, and also fighting battles, are all pretty quick in FFI DoS. I remember enjoying FFI Origins less because it just felt slower. I think FFI WSC is the best if you want smooth and fast gameplay with the original difficulty. FFI DoS is like a mix, having enjoyable gameplay like FFI WSC had, and having good translation and music quality like FFI Origins had (even if GBA can't reach PSX's sound quality).