That's the one, yep! I've also been playing the series since I was 5, maybe 4. I'm sorry you didn't hear about Mario Paint Composer until now! There are a ton of videos on youtube, and it's MUCH more advanced than the SNES version (5 note per beat limit, unlimited tempo, longer songs, files added for even longer songs, and you're allowed to pick between different sounds, and if you want, you can make you're own sounds!).
You can see some of my Mario Paint stuff in the Workshop thread "My songs". Please check it out! I can teach you how to get back into Mario Paint if you're willing to .