Not that it wasn't before, but this chapter makes it even more obvious why Himmel made all those sad smiles in the early chapters. He was cognizant of not only the difference in their feelings, but also the difference in their lifespans. Himmel did the classic proposal of getting down on one knee and sliding the ring onto Frieren's finger, all the while knowing that, realistically, nothing could ever be "eternal" between them. Frieren likely didn't understand the gesture, but for Himmel, there was still meaning in doing it. It was the most sincere expression of his feelings for her. Even if it wouldn't work out with them, he'd still always have this memory at the very least.
The dude also made sure the party got a bunch of statues of themselves made to serve as proof that they existed so Frieren would never be truly alone even after they were all long gone. He was thinking of her even without him in her life. Truly, "eternal love." How can Himmel be so perfect? He's the ideal hero.