Nice collection
@Clorgan
I really like the fuchsia, don't worry about getting the trunk diameter thick enough. Once you have a diameter over 2.5 cm, the flowers are so gorgeous that they will trump most flaws in the design. Nobody will notice flaws, they will be looking at the flowers. Similar is true of azalea, and many flowering bonsai. When in bloom, most flaws are forgiven if not ignored entirely. This makes blooming trees and shrubs easier to get to a level where people want to see them on display. Of course, bonsai is an art based requiring immense attention to detail, and one should strive for that, but in flowering bonsai there is some leeway and forgiveness that junipers and pines simply don't get.
For your spirea, twin trunk designs, typically, one trunk is dominant, one is subordinate (often referred to as mother-daughter). The subordinate trunk should be either 2/3rds the height or 1/3rd the height of the dominant trunk. Never 1/2 and usually not 3/4, as these are viewed as visually awkward. Not a "rule" per se, but it is a formula out of visual design in terms of perception.
A fun aside, in West Texas, that "puddingstone", would be considered "good dirt" because the soils of West Texas are often caliche, which has more in common with concrete than it does with friable soil. If you were to plant landscape shrubs or an orchard in west Texas, the holes are dug with sticks of dynamite, as shovels are useless. I was visiting "hill country" a couple years ago, and that's when I learned about caliche. pronounced "Cay-Lee-Chay" Just a fun fact for you. There are parts of the USA, where planting a tree in the ground is "forever", because digging up a tree, once it roots into the caliche, is near impossible, all sizing up of pre-bonsai is done in nursery pots. (West Texas is about the size of the UK in hectares).
The trident has some nice potential.
The Styrax, that trunk with the knobs, or stubbs of older branches, that is the "oldest" looking part of your tree. I would be reluctant to take off much of those knobs. Just let them be for a while. As the other trunk increases in diameter, it will balance out the look. Wait a while before you "fix" that problem area, as it does signal the age of the tree. Once the main trunk begins showing old looking, mature bark then you can reduce that area. For now leave it, as it is the only area you have mature bark.