It feels like it's been so long since I read this, but it almost feels like, you said, a bomb dropped. Not just a cruel sacrifice, we didn't just lose FL or a charming side character. It feels like a crushing beginning to an isekai, where everything from before is gone.
It looks like the villains who quickly appeared on the scene, managed to kill almost every interesting character and rendered the starting setting an alien landscape full of fearful/hateful people. At the same time, that arc seems complete in the sense that MC likely killed all of the villains from that arc.
There's also little to learn from it - like a deus ex machina scenario, there's no clear mistake or path for growth. It seems his slavery was a fatal problem that was out of his control, and even when it was gone, he just wasn't fast enough to save those he loved. Maybe the lesson is, treasure the people around you, because fate can be cruel. Not the most uplifting kind of message, but at least he was able to do what his loved ones wished for the kingdom.
It's tempting to look at this wreckage and think, like the MC, "I'd rather not face this reality any further." But I think this chapter shows him working through his grief with the kind of support he imagines his family and friends would offer. It feels surreal, but they are alive in the sense that he will carry vivid memories of them with him, so their characters live on. But their lines about starting fresh reinforce the idea that the author is taking this like a clean slate kind of fresh start.
He'll need time to tend to his psychological wounds, but then a new adventure will begin, along an unfamiliar road. I suppose the question is, will it feel interesting, or will the sadness from this part place too much of burden on the story, so that light developments feel shallow. I'm still looking forward to seeing where this goes.