Some of the ways I couldn't understand achieved the desired effect : (

ggplot() +
  geom_raster(
    data = terra::as.data.frame(x[[1]], xy = TRUE) |>
      mutate(x1 = x - 180),
    aes(x = x1, y = y, fill = lyr.1)
  ) +  
  geom_sf(data = st_break_antimeridian(st_as_sf(a), lon_0 = 180)) +
  coord_sf(
    crs = "+proj=longlat +datum=WGS84 +lon_0 = 180",
    default_crs = sf::st_crs(4326),
    expand = FALSE
  )