Something like silver birch trees don't take up too much room but will provide some height and interest. Personally I pick out natives, after asking local garden centres for those which are natural to my area. I've used black mondo grass across the front of my walls with purple fountain grass behind it. The fountain grass really takes the summer heat, doesn't need much water and self-seeds. It's grown slightly taller than me after a few years. I have, what I call a 'survival of the fittest' garden. Over the years, I've experimented with all sorts of plants. Eventually, only those that survive with little help, and almost no extraneous watering, get to stay. One of the key things I did when planting out my garden was to dig down into the soil and lay a mat of used, natural-blend carpet underlay (not the more synthetic types used these days). Just ask someone getting rid of their old carpet- it's easy to come across. Above the carpet underlay I added water crystals, then put the soil back on top. After the gardens have been established, I haven't need to get the hose out since. I do have lots of succulents around the place, some native plants and ornamental grasses. I hope this helps.

If the brick is a functioning chimney I would leave it other wise a rough render could work. One of the issues is that the nearby features such as the light and painting clash. As in the dining room, try a large mirror and a big photo e.g. to link it all. Also good rugs with strong reds, browns, oranges would help co-ordinate the rooms at low expense. I would try a colourful Moroccon style rug near the brick feature before attempting a major fix.

At this stage I would just keep attacking on the path your on. I think that right now the brick is too much but when you start to add detail, like plants, furniture, you will be happy you did nothing. You can always decide later.

once you identify any structural issues and considered advice from an engineer but i suggest that the bricks were probably never meant to be exposed and the render was part of the structure...,is it a single or double brick wall, is there an internal framed wall, what about insulation? depending on the look you prefer it might be better to add external cladding eg corrugated steel or Scyon weatherboards to allow for an air space to improve insulation as well as weather proof the building after the wall is made structurally sound