@herzausstahl - man, you have a lot of trees to work with. How do you find the time?
about ground growing - personally I am not a fan of ground growing. But then again, I'm more than 75 pounds over weight and over 60 years old. Getting down on my belly and pruning trees in the ground growing bed is not an option for me. LOL - suggestion, trees in pond baskets, grow boxes, etc. Just plunge the pot and all into the grow bed. Lift them every year or two to work on. This will keep roots pruned to the edges of the container.
A lot of the material you mentioned as being junk, I did not think was junk, just young. Developing pre-bonsai from nursery material is an art unto itself. Most of your young stuff just needs a pruning to direct future growth. Almost all young material needs to be looked at once or twice a year, and pruned, or a main branch wired, or something. If nursery stock is stuck in a grow bed, without yearly attention to its future as bonsai, it will simply become landscape material. You do need to look at it and selectively cut back when or where appropriate. I have grown out some young nursery stock, left it alone several years and suddenly realized I had missed the window of opportunity to create ''good'' bonsai. One case, was a Japanese white pine I wanted to become shohin. Left it alone to thicken the trunk. Looked at it a couple years later, and realized because I had not pruned, I had no needles with viable buds close enough to the trunk to create a shohin. Had to go for a larger size. If I had kept a couple strategic branches pruned short, to become the tree, I would have been able to remove the bulk of the top to get my shohin. It is the attention paid in the growing out phase that separates landscape nursery stock from stock raised as pre-bonsai. Make sure you take the time to assess and prune all the stuff in your ground growing bed every year, or they will get away from you and revert to landscape stock.
Thuja does not back bud on old wood, once bark develops, that area will never reliably back bud. All your young Thuja, you need to identify some branching to keep short, so you have green close to the trunk, while others can be left to grow out and do the trunk thickening. Similar with the Hinoki other false cypress (Chamaecyparis). Junipers back bud some on older wood, but not real reliably, they too would benefit to some degree with this line of care.
But I like the wide variety you have. I'll look for your posting of them in their appropriate species sub-forums.
Nice spacious yard, lots of room to play.