As your garden is symmetrical, I would mirror the plants in both planters.
Choose three foundation shrubs/dwarf trees that have year-round interest so there is always something bright and cheerful to look out on.
Trim the lower branches/leaves from these shrubs to allow lower planting underneath. For bulbs planted in between shrubs choose dwarf varieties. The leaves will take up less room and look less 'messy' once flowering is finished.
Suggestions:
Foundation plant 1:
Dwarf and compact crab apple such as Malus Coral Burst; Malus Sargentii Tina; Malus Laura.
Or Japanese spotted laurel Aucuba Japonica Crotonifloria.
Foundation plant 2:
Hydrangea - such as Hydrangea macrophylla Mini Penny; Hydrangea macrophylla Little White/Little blue/Little pink. The seedheads will provide archetectual intererst in winter.
Or a long flowering or decorative leaf hebe;
Or a repeat flowering rose also bearing winter hips.
Foundation plants 3:
Mexican Orange blossom - Choisya Ternata White Dazzler; or Aztec Pearl. You might also try Sundance which has golden yellow growth but if it has insufficient sun it will revert to green. These shrubs are easily trimmed into the size you need.
Hebe or Skimmia Japonica;
or a Euonymus trimmed in a neat shape such as Euonymus Emerald n Gold or Emerald Gaiety.
All of these shrubs have year-round interest. If you want other shrubs (such as Acers), I would plant in large pots on your patio so you can move them out of sight during the dormant season.
Underplanting the foundation shrubs:
Evergreen
Smaller hebes (many varieties to choose from), Coral bells heuchers (many leaf colours), Helleborous; Cyclamen Coum;
Perennials (come back every spring/summer)
Geraniums with long flowering season such as Geranium Rozanne (purple); Geranium Mavis Simpson (pink); Geranium × Oxonianum Wargrave Pink; Geranium Sanguinium Album (white). These will spread eventually but just pull up where you don't want them.
Bell flower - Campanulas - choose a variety that thrives in partial shade and not too spreading.
Astilbe
Annuals - plant every year and remove after first frosts,
Lobellia, Begonia, Dianthus; Asters (some varieties). Foxglove (biennial - it flowers in the second year).