Considering the reasons for why he was depressed, it's pretty understandable.
Although, he still had a good heart then, it's just not the same feeling if both that him and now him smile similarly.
i would totally read a story about a protag slime nerd who loves slimes and spends all his time researching about slimes and travels the world discovering different kinds of slimes and creating new kinds of slime by feeding them all kinds of weird shit.
feed the slime an ffing dragon or something and it becomes some epic dragon slime.
becoming the slime overlord. and creating a rimuru.
Slimes with the capability of protecting their core by dodging it sounds kinda formidable. bet that one slime could probably take on a small party of beginner adventurers
That slime better not only eat metal and slag. Otherwise it will be quite expensive to maintain a huge metal slime. Not to mention the potential damage it would create by destroying a valuable non-renewable resource. Unless this thing shits metal bricks of equal or grater value/quantity, then he should really be mindful of how many he keeps.