Using fat as a preservative,

Refrigerate them.

When it comes down to meat, you should play it safe. Spoiled meat can develop salmonella and other forms of bacteria. Read more about it on the CDC website.

That said...

I believe the method of preserving you are referring to is confit:

In chilly European kitchens before refrigeration, it was common to salt meat, usually duck, goose or pork, and then to cook it slowly in the animal's own fat. The rendered fat was then poured over the meat in a crock, completely covering it and sealing it off from the air. In an oxygen-free environment, the meat would keep a long time without spoiling.

While confit is a real thing, people from back when it was popular had lower standards for food safety, and access to safer meat. There are probably current recipes for safe confit, but a homebrew experiment of "just cover with fat" cannot be considered safe if canned - it should be handled like any other unpreserved dish.

I'm not sure that there is a way to do that with sausages. I would suggest to make smaller batches more often instead, and keep them refrigerated or frozen. If you package them in small portions, say 1 or 2 sausages, it won't take very long to thaw them in the microwave.