This volume doesn't do much to advance our hero's journey (finding a way to create a portal home), just more presents problems and new characters for him.
We get to see the limitations of Suimei's abilities. I kind of zoned out towards the end, but I know he mentioned using his Mana Oven or whatever, which is like the highest powered magic he has. So demons are pretty strong in that world, he won't be able to just fry through a million of them in one battle likely. Suimei's goal is also a bit more childish than I took him for, as is his reason to fight (I thought he'd be more about himself, which I would like, but nope, he's more altruistic and empathetic than he put on in volume 1; kind of disappointing honestly).
This introduces a new main character, a red head, Lefille Grakis, who our main character meets by chance. She's still a bit mysterious I think, but we do learn a bit of her backstory (adventurer seeking retribution for some event from her past involving demons) and get to see what she brings to the table battle wise. This series seems like it will go harem, but I guess that remains to be seen. Most of the rest of the introduced cast are more side character in nature, besides maybe Dorothea at the guild.
Besides Lefille, we do briefly see Mizuki, Tia, and our hero, along with Tia's retainers. One of her retainer's hints of a skeem to take advantage of Suimei as a decoy (I guess they threw out hints of him being the hero?) to help our hero escape. Of course, the group is having none of this, and the hero runs off towards Suimei. I believe he didn't reach him this volume, so we'll see what happens in the next one.
Similarly, our mage friend, Menia / Felmenia is also on the hunt to catch up to Suimei. She had some last second preparation (she's got some writings with advice about his magic iirc), so she's still chasing his heels. We don't see much of her after the opening, so pretty much what we know from volume 1 is what we've got.
I'm intrigued by some things that weren't 100% spelled out in this volume. One was Tia's line about misreading Suimei. I'm wondering what she took him for, and if she had read his character right, what she would have done differently. Lefille seemed more mysterious than she let on, but I think I just misread that myself. There were little wrinkles at the beginning and at the end that I didn't really love involving mindwiping and infantilising characters. I just don't like when what seems like a character relationship setup is hampered by supernatural elements that keep characters from getting closer.
The final twist involving the Demon-Lord wasn't really anything too crazy considering other series that have featured that trope. I do feel like the Demons in this series will be more sympathetic at the end of the day, maybe even the key to going home, but where will that leave our hero's party and the initial kingdom (along with the rest of the world).