Is it possible to change the difficulty mid game

The highest value items are Jewelry, especially the variants with gems. A simple malachite necklace without gem is just worth 40 but it scales with metal quality up to 2000 for a platinum necklace. Together with the modifier for Quality, prices can go far above 10000 for a single legendary gemmed platinum necklace.

However, since higher value ore is hard to get early on, mass-producing Statuettes is usually easier. A clay statuette is worth 16 and requires just one clay clump and a Kiln to produce. You can just replace all clay tiles in your fortress with dirt and get a nice amount of clay to turn into statuettes. Later on, Bone Statuettes are a nice way to get rid of the bones of all the butchered Yaks and keeps your Artisan busy.

Another alternative are Pet Rocks which can also be mass-produced and require just stone which you will have a huge surplus of anyways. They are just worth 10 a piece but due to the abundance of stone, producing 50 or 100 of them is no problem.

Your strategy of a wall around your camp and having a training yard assigned to your guards at the entrance is a good start, but here's some other things you can do to survive the first summer:

• Enemy difficulty scales with your kingdom worth, so try and keep it low until you've geared your defense squad. I'd recommend staying around 2-3k if possible.

  At around 5k kingdom worth, you start getting goblin fighters who can be geared in copper weapons and armor, and are much more damaging to your tiny gnome village than regular unarmed goblins.

• Put all your worker gnomes into other non-attacking squads, and set them to Respond to Attack Orders. Then when a goblin shows up, use the Attack button to have your entire camp jump on them. This kills them much faster, and minimizes the damage they end up doing.

  I actually had all my workers responding to attack orders for quite a while (I think 20k kingdom wealth?), until I had my guards geared and found my unarmed gnomes were dying before my guards got there.

• Keep bandages in stock, ideally at the entrance to your camp. Gnomes will heal themselves with a bandage if they can, and immediately return to their duties at full health. If they can't heal themselves (no bandages or no arms), they'll sit around in the hospital and wait for a doctor to come heal them, which takes forever.

  I don't think the hospital zone provides any kind of healing capabilities on its own... I know I've had gnomes die from bleeding out in the hospital. They seem like just a place for injured gnomes to gather when they're wounded, or to get fed if they can't feed themselves. The zone also need beds to function.

• Try to get a Market Stall built by Day 5. If you have one, a merchant will appear at sunrise and you can buy many items that could help your early game depending on what you need. Copper, Cotton (for bandages), Animals, Seeds, etc.

  To make money to buy his items, I usually recommend Stone or Clay statues since they can be made of fairly common materials early on, are relatively quick to craft, and sell OK.