Sounds like a challenge as the Alberta Spruce and Japanese maples would be my go to shrubs that are happy in containers of that size. I suspect you may want to check your soil and drainage and be honest about how much you water if both of these died on you. Container gardens can need daily watering in the summer.  You may want to consider sedum 'Autumn Joy' as a perennial and junipers such as the tall and narrow Juniper 'Sky Rocket' or Japanese holly Ilex 'Sky Pencil' for evergreen color. Add new potting soil and make sure the drainage holes are not blocked. Keep the soil moist when top one inch has dried out. You got this, Keep growing, Marianne Binetti