Nah, I haven't. Like you, I've beaten all the bosses and seen an ending and basically completed the game, but there's another ending apparently. But you have to do all the house sequences (6 in total) THEN you have to collect these 6 sun fragments. One in each Biome, but like other collectibles, they don't always spawn. So, you have to search every area in each Biome, and if they haven't spawned you have to start your run over until they do.
I get that the game is a procedurally generated one, but this just seems like shoddy design. I really can't be arsed with all that!