Any suggestions on helping it grow strong?
Plenty of fertiliser, water whenever it starts to get dry and enough room for the roots to grow. Don't prune too much until it gets to the desired size.
I've repotted it into something three times the size but still not a massive pot
That should do for now. Putting little plants in very large pots can sometimes lead to problems with roots so better to move up a few sizes at a time and allow the roots to grow some more.
It's never too early to wire shape into the trunk, the younger you do it the easier it is and the more aggressive bends you can get into it.
That depends what the ultimate plan is. I agree with this but beginners typically do not appreciate dramatic bends so early wiring is not always necessary. Gentle bends can be made when it is a bit thicker.
There is usually more than 1 'right' way to achieve the same result.