Disclaimer: There isn't much of a reason to use this mod unless you want to try the challenge it is meant for. Also, there are files for 1.20pre and 1.19.8.

This mod doesn't do much except tweak a few things to allow for a decent amount of progression, without adding new content or the like. 

Changes:

1. Made it possible for peridotite, bauxite, and limestone rock layers to spawn. These layers do not spawn in small words normally for some reason, but with these changes, you can reach the Steel Age. (Note that they still aren’t guaranteed to spawn, so keep generating new seeds until you find a good one.)
2. Increased the spawn chance of borax and iron. Simply to minimize the chance that you have to pass on a viable seed because it lacks these. Oops, it just caused a ton of them to spawn when they can spawn, oh well.
3. A Handful of changes to the treasure trader, notably switching around a couple of it's selling items to buying and decreasing the time to restock to a single day, and removing it's claim protection as space is limited.
4. Some adjustments to flax such as reducing the growth time to 3 days (based on default days per month) and ensuring that a second seed drops, and increasing the trader's demand of sewing kits to 999.

The Actual Challenge

If you do wish to try the challenge, the only requirement is that the world's length and width be under 8x8 chunks(256), 4x4(128) is recommended though. You can freely change any settings aside from that, for example changing copper and tin's surface frequency to very common- which is also recommended. Then you'd need to find a viable seed, or use one of the ones i've listed below. You'll have to rely on a treasure trader to make it far, if it wasnt obvious from a couple of the changes above.

A viable seed would need a trader, gravel/sand to pan, all rock layers(periodite, limestone, bauxite), all ores(copper, tin, iron, borax, olivine), and preferably cattails- as you cannot get any more cattail roots other than what you start with. Once u have such a seed, regenerate it with the same settings until you have a treasure trader, the trader will automatically spawn as a treasure trader.

Viable seeds for 128(4x4 chunks):

• 1195216975 - Flax, clay, beehive, but low on cassiterite and spawns some wolves and a bear
• 1049488467 - If you want more of a challenge, no flax, clay, beehive, low on copper & cassiterite, but lots of gravel
• 985913473 - Extremely challenging. Just a barren world full of gravel with just a trader, 2 resin trees, 2 ponds, and a few trees. Working on some changes to help with such a start without circumventing the challenge.

https://pastebin.com/xG14EBy2 are my settings for the above seeds, feel free to change it as you'd like. You can lower the forestation, but lowering it on 1195216975 will cause the beehive to not spawn. For the third seed set forestation back to normal.

Viable seeds for 256(8x8 chunks):

• 1968243335 - Flax, clay, 2 beehives, low on cassiterite

Use the settings for 4x4, and change the world length/height to 128.

Here are the settings i use for seed hunting: https://pastebin.com/qgGpKbSZ.

Look for a trader first, all necessary rock layers & ores, gravel, then look for additional things like cattails, flax, and clay. Cattail tops, flax seeds, and clay can be bought from the treasure trader's vessels, so they aren't strictly necessary for a viable seed. When u find a good seed, increase the forestry value til u see resin and potentially a beehive, keep in mind semi-arid should be kept as that gives you a good chance to get soil and gravel. 

If you find a viable seed and want me to add it here, comment the seed below with your settings in a pastebin.