does anyone have any other post apocalyptic manga starring people who think theyre the last people alive? if not that, then any manga with gigantic architecture
Well I don't know if this fits your description but Tasogare-Boshi no Sui to Neri is a short manga about a girl and a talking sloth living their lives on a Earth that is slowly declining. It isn't exactly bleak like Girls Last Tour though.
..And since I'm recommending that, maybe give Rojica to Rakkasei a try? It's about a girl who seems to be the last human around, interacting with animal like creatures in the world.