I really enjoyed reading your thoughts! And I think I feel the same way about the book. I liked it a lot, but it was too short to be the kind of book that picks me up and sweeps me away the way JSMN did. I also enjoy historical fiction, so I initially started reading thinking Piranesi was going to be historical too, but then we got to the bit with the Family Circle biscuit tin and I thought: "huh".

Piranesi himself is most definitely a sweetheart! He's adorable. I didn't really enjoy the ending myself, because I had come to like Piranesi so much that I really missed him in the end. For the ending to work for me, I think it would have to be quite a bite longer, so we can get to know not-Piranesi too.

Your point about Arne-Sayles being an outsider is really interesting, and I think you might have hit the nail on the head about why he was written the way he was. At the same time, it was pretty tone-deaf to make one of the only queer characters the villain, so I struggle with it. (Honestly, I thought Piranesi was going to be written as gay too, so I was very surprised when that wasn't confirmed.)

There's a lot of JSMN fandom going on on Discord at the moment (it also has a specific channel for Piranesi spoilers). It would be lovely to see you there if you fancy joining. Here's the link:
https://discord.gg/ymnRNV
The link is only active for a day, so let me know if it runs out and you need a new link.