Open the scn file as described here: https://steamcommunity.com/workshop/filedetails/discussion/1693186543/1846946102859416839/

for the flatlands it is this file:
\steam\steamapps\workshop\content\858810\1693186543\Flatlands\flatlands.scn.xml

- Animal Values:

Search for "animal" you will find those lines for the animals: <action type="SetAnimalPopulation" animal_types="wolf" min="15" max="20" era_factors ="1 1 1 1 1 1"/> adjust min & max, the era_factors are for less or more animals in specific eras.

1.3 NEW ANIMALS: cave_hyena elasmotherium muskox feral_horse
Paste them over or under the animals.
Example:
<action type="SetAnimalPopulation" animal_types="cave_hyena" min="17" max="35" era_factors ="1 1 1 1 1 1"/>

- Trader:

Search for "trader" you will find this line: <action type="SetTraderPeriod" value="2.5y"/> adjust the value in ingame years. ("0.5y") to have 2 traders per year. Increases with prestige.

- Disease:

Search for "disease" you will find this line: <action type="SetDiseaseParameters" period="2.5y" variance="0.5y" individual_disease_chance="0.5"/>

period in ingame years, variance adds variance. happens between 2 and 3 years with this example. not sure about individual_disease_chance="0.5" the value might affect that there are multiple diseases (f.e. barley + einkorn + sheep)

- Storm / Blizzard:

look at the top of the file:

<disasters>
<disaster disaster_type="Storm" period="2.5y" variance="0.3y"/>
<disaster disaster_type="Blizzard" period="3.5y" variance="0.3y"/>
</disasters>

parameters work in the way the do for diseases.