thanks for the detailed feedback everyone - looks like I’ll strip the bark off the dead part of the trunk and let the tree grow out for a while in a bigger pot. also @Shibui, the wire at the top is for one of the smaller upper branches in the canopy - i intend to use a thicker wire for the trunk itself. @Esolin really digging that style and think i’ll try to achieve it! Any recommendations on the type of soil for growth? a fast-draining ~20% organic mix maybe with the fertilizer you mentioned?
I would certainly recommend adding some organics in your mix. Loose, free-draining is ideal for bonsai, as the roots don't suffocate--they need both water and oxygen to reach them. But sometimes the stardard mix can be too free-draining for a climate. You're in So Cal, yes? So am I. Here it's very arid, and Summers are hot and breezy. These three things really dry out potted plants, and the smaller the pot, the faster they dry out. Junipers can take drying out to some extent, but again, a small tree in a small pot on a scorching Santa Ana day could fry to a crisp while you're away at work all day. Game over.
Depending on your local climate and the particular place your tree spends its days, you will need to make adjustments. You can use a mix that is richer in organic, moisture-retaining components like pine bark or spagnum moss to help compensate. You can use a humidity tray. Or shade cloth, or any combination of strategies to keep it from getting cooked in the summer. I don't have auto watering, and I'm gone all day at work. All my smaller plants get afternoon shade in a wind sheltered location. And I mix some organics in with pumice and akadama to slow out the drying.
For fertilizer, organic types are safer since it's harder to burn your plant (again, small pot = less wiggle room on ratios) and plants generally do better with them long term. But you can use chemical sorts like Miracle Grow, just keep the mix very weak, or only use them occasionally to supplement organic types. Also, don't fertilize at the height of Summer or dead of Winter. Fertlizers are basically mineral salts, and a heat-stressed plant will struggle even more to get enough water if there's too much 'salt' in the soil. Could be fatal. This is why they say to never fertilize a struggling/stressed plant. And plants mostly stop growing during the coldest months, so it's kinda pointless.
Good luck with your bonsai!