I'll be honest, I've read a ton in this weird little niche. The stand-out for me has to be 'Warm Bodies' by Isaac Marion. It starts with a zombie eating a boy's brain and then falling for his girlfriend, which sounds utterly bonkers, but it's handled with this surprising depth and lyricism. The character arc for R is incredible; you watch this glimmer of consciousness return, piece by piece, as his love for Julie literally makes him more human. It's a metaphor that could have been clumsy but ends up feeling profound.
For a grittier, more desperate take, 'The Girl with All the Gifts' by M.R. Carey fits, though the romantic element is more subtle and maternal. The real star is Melanie's arc from a sheltered, brilliant child to a being with the power to decide the future of her species. The relationship between her and her teacher, Miss Justineau, is the heart of the story, full of fierce protectiveness and horrifying moral choices. It’s less about kissing and more about the different forms love takes when the world is ending.
A recent find I devoured was 'Hollow Kingdom' by Kira Jane Buxton. The romance is between a domesticated crow and his dog best friend, set against a zombie apocalypse seen through animal eyes. The protagonist, S.T., has a massive arc from a spoiled, human-centric pet to a reluctant leader of the animal kingdom. It's hilarious, heartbreaking, and unlike anything else. The love story is about found family and loyalty, which in a post-collapse world, hits just as hard as any human romance.