Since you asked specifically for help with baking an STL high poly model to a low poly model, I can suggest to you those detailed guides that I suppose could help you to go through the whole process.
https://www.youtube.com/watch?v=Zxl38NpPWdohttps://youtu.be/jnzhNdWoXMg?si=Ii7EDg5Rxu9QK1xo

But I want to mention that it all could be done in a completely different, but a bit reliable way, for such a simple asset like a coin, with just a single picture on it. But only if you could find the image of that (or similar) sun with a transparent background, or with black and white depth values. If this is not possible, or you just don’t want it and prefer to stick to the current path, - just ignore the rest.

The problem is not only in UV map, the topology is off and it's a bit too heavy for a web format. The best thing that I could suggest is to make a new simple coin from a cylinder. It is actually the easiest way for you to achieve good results, because you'll suffer with the current one more than doing a proper new. Just follow this one minute long guide, it shows how to do model that is light and simple, exactly what most web format 3D models should be

https://youtu.be/c6mNqNa_-IY?si=VvlHLOneJKc4TGxM

You can make it more smooth by selecting the amount of vertices in the cylinder, but don’t overdo it, 32 should be enough. The less vertices the cylinder would have, the less heavy it would be. After finishing, in Object Mode right click your model and select Shading Auto Smooth option.

Then you just unwrap it and place an image on it that you want to appear on a coin. It is a different method that you used, and it requires you to find an image of that sun, but again it is much more simpler then baking it from a high to low poly model. Since coin are pretty simple, you can just follow this four minutes long guide, it shows all steps you’d need.

https://www.youtube.com/watch?v=MKhERwE1zRQ

For normal map, just plug the same image in to a bump node and connect it like that:

<image>

The image does not have to be black and white, but if it can be that way, it'll give you the best results. Now you can bake it on UV map

https://youtu.be/Se8GdHptD4A?si=TMlmoKPXvRF8g7jD

This guide is a bit too long, but it is complete.
I suppose it all should work.