Just my two penneths worth, but try to do the split faced tile in a random bond so as not to draw the eye to individual tiles. The small wedges are good to take up variation as they can vary in width and being square. But if you haven't got any then use some thin cardboard. Goodluck with the project. All the best
Those tiles are 8-18mm thick slate....there's NO way I'd be putting them onto that wall without mechanically fixing a backerboard such as Hardibacker to it. A plastered wall will only take 20kg per sq mtr....thats for tiles and adhesive etc. You could have mechanically fixed these without scraping back all the paint. If the tile boxes don't have a weight printed onto them...stick a couple of boxes onto the bathroom scales.
I'd be mechanically fixing backer boards too. Skimming plasterboard or render ruduces recommended hanging weight 20kg per m2. These tiles are much heavier