Flash fill usage in harsh sunlight is not at all complicated. It was originally called the Synchro-Sunligh method. There was a limitation with the focal-plane shutters but if you have HHS you can employ the full range of the system in that the fash will synchronize at high shutter p speeds.
The basic rule of thumb is EXPOSE FORT HE FLASH AND SET SHUTTER FOR HTE DAYLIGHT. Here is a typical example usg the OLD f/16 Sunny Day formula: ISO 100 - flash GN (ft.) 160 - 10' = F/16 Shutter 1/00 1/125 sec. Lighting ratio 1:2
You have to know your flash exposure by GN or meter in order to arrive at proper flash exposure.
Of course, if you are concerned about diffraction, you might NOT want o use f/16. if you prefer a wider aperture you can adjust the flash output and/or the IOS settgs accordingly. Once you master the concept it is an easy seting to arrive at.
I learned this method many years ago as a weddg photographer. There were and still are many occasions where I had to work just after the ceremony outside the church or at an outdoor reception in noon-time direct sunlight. I had to shoot lots of fast candid shots so I
simply set the aperture for the typical sunlight setting and fire away. As long as I knew the flash flash was no go to overpower the sunlight I always get a good ratio with transparent shadows.
If you are working in open shade or low light, or want to use a very wide aperture for limited DOP, you can still employ this method all long as you can power down the flash. Current speedlights have 1/8 or lower power settings.
As for issues such as "raccoon" eyes, fill lighting and ratio control is the only practical approach other than avoiding direct overhead light. Trying to address this post-processing is tedious and usually not successful.
There are othere fill methods that utilize reflectors but for fast hand-heal shootg with on-camera flash, this basc formula as explained above has been a stand operating procedure for press and wedding shooters over the years- it's kinda old school but it works.
In my own equipment situation, for fast hand-held shooting, I don't use a conventional "Speedlight". I hace several portable strobes that have separate power packs (shoulder packs) and a lamp head wot interchangeable reflectors. I can reduce the power to a "wink light" with a reduced out to 5 watt-seconds, remove the reflector (bare bulb) which cute 2 f/stops, or run up the powere to 200 watts. seconds and fill a gorp at 15 or 20 feet away. Or, I can employ my "famous" "Shapiro Flash Valve"- I put my hand in front of the reflector and just let a sliver of light escape through my fingers.