I managed to 100% the game, wooo! I love all the details in the game, and the collectibles are really fun to get! The final level is tough as all get out but after roughly 150 lives I managed to get through. Tip for grinding lives: Go to the first underground segment, go to the right, and get up the wall. Three lives will be there waiting. Then pause and reset the level. You will keep the collected lives and be able to get more. This is probably what got me through the last level.
Great work on the game, and I'm happy to see a Little Runmo game! Really cool!
Good luck to everyone who plans to do the same!