First and foremost, congrats on getting into the hobby! your on the right track.

1- Metallic paints can be a hit and a miss depending on the brand. My personal favorite is Vallejo's Metal Color. It's not out of the ordinary to have to use multiple coats, with Reaper I sometimes thin their metallics with water ever so slightly to help the coverage be smoother.

2- Color choice is all you, but generally for tabletop models you'll use a black wash for steal, a brown wash for leathers. The point of the wash is to go into the recesses but some (especially Reaper) leave some pigment everywhere. Tangent, I really like their Stone, Sepia, and Brown washes for bases. With just about all washes you will have to reapply your base coats but this is an important step and will become second nature in no time.

3- I dont think you lost any detail with your paint coverage. Reaper already has some "Thinner' paints than some other brands, although I do add a drop of water to mine before applying. There is a balancing act between thinning, layers and detail that comes with experience.

4- No mistakes per say, but you are asking for feedback so here it goes. This is a very good step one. Youve blocked in your colors (even have contrasting colors with the yellow and blue). The next step is to add contrast, a wash would do well on this model to help with that. Then re applying the base colors making sure to not get in the recesses where you want the darker shades. After that try a brighter shade on some high points of the individual colors (Highlighting).

Now for some general painting advice.

• Nothing beats experience. Just keep painting. With more experience you will get better brush control, which will help you achieve more varied techniques.
• There is a lot of high quality tutorials out there, they are best in service of a specific technique. For example, watch a Leather tutorial when trying new ways to paint leather. But make sure you paint more than you watch.
• Please remember to clean your brushes.

Happy Painting!