All of those shots look underexposed to my eyes.

Stuff to keep in mind with flashes

Bigger is better, closer is better.

The bigger the light source is, the softer it appears. Same with distance.

Also, the closer the flash is the less power it needs to achieve the same brightness.

Being able to get great shots with an on camera flash with a bounce card or similar is a good first step.

A ring flash adds a bit more flexibility, but the real gold is when you get the flash off camera and start moving it around (and add extra lights) .

The goal of shooting with a flash is usually to make it look like you didn't use a flash, and this is true for whatever situation you're shooting in.

The best time to use a flash is when most people wouldn't think to - bright light outside. It'll help even out harsh shadows from overhead sun.

There's a few different techniques to familiarize yourself with, as well as when your flash fires.

Front curtain sync, rear curtain sync are the main ones.

In most cases, rear curtain sync is preferable, as it'll lead to any motion ghosting being behind the subject. Front curtain sync leads to motion ghosting being ahead of the subject which looks weird.

Using flash as the main light source is different than using fill flash to bring up shadows and balance things out. The camera settings and metering will be a bit different. Simply pointing an on camera flash at the subject and taking a picture usually ends up in a DMV style horrible photo. Don't do this.

I could list all sorts of crap but the best thing to do is to experiment and ask questions. Post pictures, explain what you were trying to do and what you did, and we can help you a bit better.

Also worth noting on TTL exposure, it's never and I mean never going to be consistent between shots.

Kinda like auto WB. The camera will always decide to fiddle with something.

Don't expect any consistency. The newer brain in the D500 is better at this, but it's still not perfect.

If you're shooting with the flash as the primary light source, shutter speed doesn't really matter anymore. The flash becomes your new shutter, and it cycles far faster so it can freeze motion a lot better. Edit - this is true up until you're at say 1/2 to 1/1 power - at this point shutter speed *may* become a factor depending on your specific flash. Some are slower than shutters at this point.

If you're doing fill flash instead, shutter speed does come into play when balancing ambient light, so stuff like high speed sync (Auto FP mode) will come into play here.

Flash with HSS will never freeze motion as well as raw low power flash can. Generally speaking, flashes at their low power setting can cycle in the 1/30,000 of a second range or so +/- a few thousandths either way . All adjusting flash power does is change how long the flash is on, not how bright it is.

The trick to good exposure and good motion freeze is having multiple flashes close to the subject firing at low power to get the fastest cycle times.

Everything you want to know is here : Strobist

One of the things that took me a while to wrap my head around was fill flash. I thought it was complicated, but it really isn't.

Expose like you aren't using a flash, then let the flash pop (even auto works for this) to lift the shadows a bit.