How should I thicken the trunk of this Larch?

[one_bonsai]

I was thinking of wiring the trunk and then letting the leader and the lowest branches grow long as a sacrifice.

[shibui]

Your plan is good.
A couple of things to be aware of:
The leader tends to thicken the entire trunk evenly, lower branches thicken the lower part to make the base even thicker.
Strong leader growth can suppress lower parts. You may need to prune the leader a couple of times to get lower branches to grow well. Always monitor trees where you have a strong sacrifice branch, especially when it is above the rest of the tree.
Watch your wires. If you are successful the wires will need to be removed sooner - before they mark the trunk.

[one_bonsai]

You may need to prune the leader a couple of times to get lower branches to grow well.

How do I know when to prune the leader? When it reaches about two meters?

[shibui]

There is no rule about how high the leader should get. It is far more about the desired result which is your trunk down below.
Prune the leader:
1: When the trunk is nearly as thick as you want it to get
2: When lower branches start to get weak or are not growing well
3: When the tree gets top heavy and keeps falling over.
4: If your trunk has no taper or starts to get reverse taper.
Note that growing sacrifice branches is not a once off thing. Even if the trunk has not thickened enough you can cut a sacrifice branch back if needed. Usually lots of new shoots will grow and you can let one or more start to grow long as new sacrifice branches. This cut and grow process will help with 2 and 3 above.

[Sno]

Hi one . Just something to bear in mind with Larches , it's important to have buds lower down and close to the trunk . Larches don't reshoot on bare wood so where ever you cut you need a bud under the cut for it to keep growing again .

[one_bonsai]

Would you do trunk chops on larches like you would with say an elm?

[Sno]

No and yes , you can trunk chop them hard as long as there is a bud below the cut (with elms you just chop them and buds usually pop out of the trunk .).
I am not sure how they go at grafting ,you may be able to graft new growth low on the tree if you don't have a suitable bud and once that is growing strongly then chop down to above the growth . I have never grafted Larch but someone here may have .
With your tree you can chop down to the lowest branch then wire that branch up as the new leader ,grow and then repeat .keep as many buds low as possible while you are doing this ..