Trivia - there is more than one way to control ambient light !

1/250 f/4 iso 100 1/2 power flash (1 stop down in shutter speed, 1 stop down in aperture, 1 stop higher in flash power)
1/250 f2.8 iso 50 1/2 power flash (1 stop down in shutter speed, 1 stop down in iso, 1 stop higher in flash power)
1/125 f5.6 iso 100 1/1 flash power ( 2 stop down aperture, 2 stop higher in flash power)
1/125 f/4 iso 50 1/1 flash power (1 stop down in aperture, 1 stop down in iso, 2 stop higher in flash power)

The above assumes that your trigger is fully synching up at 1/250s, no black bar banding and you dont need the full dynamic range of the sensor as using ISO 50 reduces the dynamic range of the shot (1 stop overexposure and bring in down similar to lightroom but done in camera).

My main concern here is what is your lighting setup and how are you filling in the shadows. If ambient light is affecting the shadow fill, reducing in by 2 stops withing putting any fill on it my result in undesirable high contrast look. Pye Jirsa in SLR Lounge's lighting 201 highlights this, stating that controlling ambient exposure via shutter speed can be used to control the fill side of the subject, particularly useful with 1 light setups on location.

 

Thank you @anonzzz tutorials on p-v is helping me a lot, before I was clueless on flash photography.

Yes a lot of the youtubers only share basic knowledge with a large number of them not knowing the full extent of flash photography.

Oh yeah I forgot to mention we can also bring the flash closer to the subject instead of increasing flash power (inverse square law of light), how did I forget that.

You are right you don't really want to adjust aperture too much as it affects the artistic look of the photo (bokeh - dof) which is usually important for portraits. However, in some cases shutter just has important role, you may be using shutter drags in portraits where you cannot afford to adjust shutter speed too much.
Increasing flash power also has its consequences, longer recycle times, longer flash durations (t.1), this must be factored in, and if you are on location using battery powered flashes like AD200 and need to take many shots and have only 1 battery, this must be considered by using lowest flash power as possible.

Generally ISO is the safest to use after shutter speed but it must be kept as low as possible to maximise dynamic and keep noise to a minumum.

I guess you have to asses the situation and make an adjustment which has the least negative impact on your shoot. Bringing the flash closer will give a quicker light fall off, which makes it a more dramatic picture, but you wouldnt want to do that in a light and airy shoot.

 

Yes you are correct, I am starting to overthink and over complicate it again

 

I have a question regarding flash actually, when you are free you can take a look at it When shooting groups and using sunlight as a rim light, do you use light meter at all? Do you measure the rim light, ambient light (fill) and key light (strobe opposite to sun light). Or do you just go by the looks of it in your EVF/LCD? If you measure it, what ratios do you typically go for? What lighting patterns on faces are you aiming for, I am thinking loop-Rembrandt since the umbrella is placed to the side it will be hard to get exactly the same pattern on each face on the group at the sametime trying to avoid shadow casts by people closer to the light.

 

Good point I guess it also depends on what style you going for, dramatic lower key look or light and airy higher key look. I would also say point the centre of the light modifier (umbrella) to the person furthest away from the group to get a more even light distribution (inverse square law of light)