After lots of tinkering, plus countless hours of trial & error, I've finally got everything working just how I want it.
So to answer my own question - "Can Stringify work directly with Home Assistant" = Yes it can!
It was always my intention to have Stringify be the main brains behind everything & thankfully I've achieved what I set out to do, I've reduced the number of cloud servers I was using.
I'm 100% sure that Stringify is now the only cloud-based service I'm using in my current automation's, I've been using Stringify for nearly a year and I don't think I've ever had an issue with the platform, the majority of issues where other servers & not the Stringify app.
Stringify is a fantastic platform, its such a shame there is no direct integration for the Sonoff devices, using IFTT as a workaround does work & its "kind" of reliable (like 95% of the time).
It's still too early for me to say if swapping all my Sonoff's over to Home Assistant and using Stringify to trigger the scripts/automation's is going to be more reliable but I can tell already that the response times of the devices have reduced massively. When using IFTT it could take a few seconds for devices to respond, now its near instant.
One of the biggest issues I had was finding how to get Stringify to call individual scrips from Home Assistant so I'll pop the info in this thread in the hope it helps someone else in the future (I couldn't find it on any forums and it took me the best part of 2 days to figure out, peacing together little snippets of info from different threads all over the internet) .
The method I posted above will call all the scrips in your scripts.yaml file, all of them at once.
To get Stringify to call individual scrips in Home Assistant you need to create the script, then reference the unique 13 digit number that will be listed in your scripts.yaml file.
Use the same URL posted above but replace the "turn_on" (keep the ?) with the 13 digit number from your scripts.yalm file, no date, no authentication type. That's the first method I've found that works and I'm not sure if its the best but at least it's working.
Also, if you wanted Home Assitant to trigger a flow in Stringify you would use the Rest_Comand component in Home Assistant, the instructions for that are easy & straightforward.
I hope this information helps others like me who love Strinfiy but also have a home full of Sonoff devices.