Baking bread with a firm texture that doesn't grow mold

Mold grows on bread because mold spores in the air, or on hands or other surfaces, get transferred to the bread. Bread has plenty of nutrients for these spores to grow. Mold growth is enhanced by temperatures over 70 degrees F (21C). Humidity and moisture also favor mold growth.

There are a couple of things you can recommend or do to reduce mold growth.

Bread should be handled with clean hands.

Bread should be kept well wrapped.

Refrigeration dramatically slows mold growth.

Freezing virtually stops mold growth.

If not freezing, consume bread as soon as possible.

Margarine and milk help keep the texture soft, and reduce staling. If you want a firmer texture, you might consider a formula without these ingredients.

Moist breads will get moldy faster than dryer breads, and moister breads also are more open, i.e. a have spongier texture, so reducing the milk in your recipe may help resolve both of those complaints. I would suggest you reduce the milk by 5% and see how that improves the situation, then keep reducing it by 5% until you get the texture you want. Reducing the moisture will make the bread less open, if it gets too tight add a bit more milk.

Take notes on the changes you do so you remember what works and what does not for your recipe.

There are answers about how to use preservatives in bread here.