The first thing I would do is to paint all those vertical surfaces, trellis included so they look the same. Dark charcoal is v fashionable atm and makes a good backdrop for plants so your eyes see the plants not the trellis or fence. It would look good with your table as well.
You have some nice plants in pots but they are badly displayed. Try to get hold of some bricks or breeze blocks so that you can raise up the height of pots at the back, then play around with them until you get an arrangement you like. You already have some quite tall plants, raising the pots will help with the illusion of even more height. Put plastic pots at the back, nicer ones at the front! B and M garden centres do very reasonably priced stylish pots if you can afford a few new ones.
I find that most herbaceous border plants will do fine in pots as long as you water and feed them regularly, just keep an eye out for pests. Agapanthus, lemon verbena, sedums,geums, New Zealand flax, grasses, cranesbill geraniums, verbena Boneriensis,
Many herbs ,bay, rosemary, thymes, mints, sages (edible and decorative) bronze fennel have lovely textures and colours and are often evergreen for all year interest.
In the summer you can plant out bedding plants in pots, but at the moment it is bulb season. Google “lasagne planting” to get the most out of bulbs in pots.
The advantage of pots is that whereas in a border you often need to plant out a number of the same plant to make an impact you can get away with just fewer plants to make an impact.