You'd have to close the lens down to compensate for the slower shutter setting giving you more depth of field.
My example has us at ISO 100 which is rock bottom on some cameras.
You could use an ND filter on the lens to compensate too, but I like being able to SEE what I'm shooting through the viewfinder.

BTW, Bokeh is the quality of the blur, not shallow depth of field.
It's a Quality, not a quantity.
There is good and bad bokeh, not more or less.
More HERE.
The lenses I use have fairly pleasing bokeh so I really don't worry about it.
There are better lenses out there for that, but I'm not a full-time portrait photographer and can't justify the expense for one.
And there are more important things most people should think about (unless you have a real lemon of a lens) like lighting, pose, impending bad weather....

Back to flash... Your example using flash looks great for a flash newbie.
My biggest thing at the university where I work is shooting events.
When I use flash, I almost always bounce, but because of the way I use it, it doesn't look like flash.
I've bounced off of cement headers over doors, metal garage doors and 20' ceilings.
Neil van Niekerk has a lot of tutorials on bounce flash starting HERE.
Also has some helpful books on his site.

Re: High Speed Sync (HSS)...
One thing it allows you to do is use flash outside with a wide aperture to help keep that background pleasantly out of focus with portraits.
Let's say you're shooting a portrait and your ambient exposure at iso 100 is 1/1000 @ f/4.
You need to add a flash for fill though.
With a normal sync setting of 1/250th, you'd have to use f/8, bringing that background to a distracting level.
HSS (or Auto FP in Nikon-speak) lets you keep the original f/4 exposure.
Yes, it takes more flash power because the flash pulses instead just of one pop, but...using a wider aperture requires less power anyway.
If you need more light, you can add another flash ( there are special brackets HERE and HERE that hold more than one) or move it closer if possible.

The shortest shutter speed your camera will fire the flash at and still illuminate the entire scene. If your camera says it's 1/250 of a second, then you can't go shorter under normal circumstances see the end of this post for when that rule doesn't necessarily apply.

This is what happens. Your shutter has two moving parts that create the image - a front curtain and a rear curtain. When you use a shutter speed that is longer than 1/250, the first curtain opens then the second curtain closes. The cumulative time that the entire surface of the sensor is exposed to light is then 1/250 second or longer. When you use a shorter shutter speed, as the first curtain opens the second curtain is following it, creating a "slit" that moves across the image, with the end result being a shorter exposure. The slit gets narrower as the shutter speed gets shorter. Because it is a slit, and the duration of the flash at full power is probably around 1/900 second, going down to 1/4000 or less when you use a lower power mode on the flash - there is no point in time when the entire sensor is open to receive the light from the flash pop.

High Speed Sync is a mode that will let you shoot at shutter speeds shorter than 1/250 and still give you fully lit scenes. It does this by commanding the flash to do very fast multiple pops, so that there will be light at all points in time that the slit is moving. The downside of this is that your batteries get eaten up quickly, and your flash to subject distance is diminished.

https://alphauniverse.com/stories/the-power-of-high-speed-sync/

A flash bracket is a good idea if it is not too obtrusive, but you are wasting time on a diffuser - most don't do much if your subject distance is greater than 3-5 ft. If you are using a Fong thing, then you'd better have a small room with low ceilings, otherwise it behaves like a point source of light, and you will have harsh shadows and contrasty light. Bouncing, whenever possible, will give you the best quality of light.

Now that you know the fastest shutter you can use, you can address "how much slower should i use"
How much subject isolation do you want?
What color light source balance is involved?
How much will the flash alter the mood.
Is the flash for illumination or freezing of motion?
How will the flash effect the appearance of the background?
Will the flash help you recover the true colors?
Are you using the flash to match the light coming in through a window?
Will your flash irritate the snot out of people trying to watch the show? etc...etc...
Have fun experimenting.