The TT600 is a single-pin manual-only speedlight. It has no TTL capabilities for the camera/transmitter to control. You can only use it in M. And you can only adjust the power in M from the transmitter (though you could I suppose set the Group to TTL; it will only fire at full power that way, though).
Cameras react different to having flash or not having flash, but the XPro-P on the camera hotshoe should have acted as if you had an on-camera speedlight. But Auto ISO is never really a great idea with flash, because of how cameras tend to react to thinking you've got some ambient settings that are really really dark. :)
This is just me, but honestly, a TT600 isn't the greatest choice for a first/only speedlight, no matter what David Hobby writes. It's better as a 2nd, 3rd, or 4th off-camera only speedlight. But you want at least one unit you can use on or off-camera. Or at least in TTL. The V1-P, V860 III-P, or TT350-P are your only Godox choices as a Pentax shooter (unlike all the other brands who also get $110 TT685 options). But you could, if you plan to use this flash off-camera all the time) have also used a TT685-C, firmware upgraded to speak P-TTL off-camera (its foot remains Canon-dedicated, though), and gotten TTL control over your off-camera flash.
First thing to wrap your head around is that whenever you take a flash image, you're combining two exposures in every shot, because you have two different sources of illumination: the ambient (all the existing light that isn't the flash) and the flash. Ambient exposure is controlled by iso, aperture, and shutter speed. This you should already know and be comfortable shooting in M before you start up with flash, because the automated system in your camera uses a meter that can only read the ambient light (what's in the scene) before you take the shot. It cannot measure your flash because the burst hasn't happened yet. The reason I say TTL makes it easier is that with TTL, the camera can put the flash into the scene while it's metering with a "preflash", and then adjust the power on the flash to match your other settings. But it can only do that with a flash that speaks P-TTL. Flash exposure, however, is controlled by iso, aperture, power, and distance. Shutter speed only affects the ambient because the flash burst is much faster than your shutter speed, so leaving the shutter open for longer only gathers more light for the ambient.
Because of these two different sets of controls, you can have two different exposure levels in each image based on the two different light sources. You can balance your flash against the ambient at any combination that's within your flash's power range.
When you rely on automated exposure modes (aperture or shutter priority) on your camera, the camera is most likely going to default to "fill flash"; where most of your exposure comes from the ambient, and only a little from the flash to "fill-in" shadows in bright sunlight. This is a very common use of pop-up flashes and what the camera is designed to default to. If you want to change this, you typically have to be in either the M or P modes on the camera.
Also there is an additional "gotcha" when it comes to shutter speed, which is that every focal-plane shutter has a flash sync speed limit. For most digital cameras, it's around 1/200s. With a K-70, it's 1/180s. Go faster than this without engaging high-speed sync (HSS) mode, and you'll get banding because one or both of the curtains will still be covering part of the sensor when the flash burst goes off. If you do engage HSS, then the flash has to pulse to form a continuous light during the whole of the exposure, and you lose about -2EV of flash power to this. So most folks stick at or slightly below sync speed if they can, to conserve power.
A very typical trick to do flash outdoors with someone is to drag them into the shade, meter and slightly underexpose for the ambient, and then add the flash to light them a little brighter so they "pop" in the frame. But what power setting to use depends on what iso and aperture you're using, how you want to balance the flash against the ambient, and how big/powerful your flash is. If I'm inside, I tend to ballpark iso 400, f/4, and 1/4 power at sync speed, simply because mnemonically it's easy to remember all-4s. :D Then I chimp, and adjust. With experience, you get to sense what range of power you may need for a given set of settings.
But, as I said, TTL can be a lot easier, particularly at first. You do also need to learn to operate a flash in M, though, just as you need to know how to operate your camera in M, so you can get a sense of what's actually going on. Both Neil van Niekerk's Tangents website, and David Hobby's Strobist website are great places to start.
I typically say start with Tangents and learn on-camera bounce flash first, since it only involves buying and learning a speedlight at first. But the lack of TTL on a TT600 might make that a bit more frustrating than it would be with a V860 III-P ($230) or TT350-P ($85), and the TT350 only swivels 270º which is a completely different PITA for bounce flash. So, possibly starting with the Strobist's Lighting 101 is a better starting point for you.
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