To thicken the trunk, you simply need to let it grow. A larger training pot would help. You could slip-pot into something a bit larger. I’d also be cognizant of what kind of trunk movement you will to want in the future, now.
You’ll just need to do your research on fertilizers, as it’s as controversial as soil mixes and substrate types. I was recommended to use something on the mild side, specifically 5:5:5. I went with Dya-Gro 7:9:5.
Any time the wire is biting into the tree’s truck or branches, it’s already past time to remove the wiring, unless intentionally going for that look. I think some do that with pines, if I recall correctly. I’d go ahead and remove that wire, as was mentioned above though.
What makes this hobby so great for beginners, is that you have a hobby that requires patience for growth which in turn allows ample time for researching your specie and everything bonsai. You should have plenty time to study up on this little guy.
I wish you the best of luck, and keep us updated on your journey!
