Thanks everyone for your posts! … Hopefully together we can all work out how to get this working better. 
I see today Epic have released their roadmap which also allows us to vote on the most useful features we would like to have.
Of really great interest is the suggestion they are asking us all to vote on, is if we would like to have “Mobile preview in editor”! … that would help a great deal as we could get on with more development, (especially now), while we wait for better overall support of older phones. … It is these build times I’m seeing here, that I was posting about, that is a big problem for actual devices currently, but if we can use our PC’s as another target device effectively, for mobile development, then that would greatly help us, until they have more time to better support these older phones.
Here’s the link if you would like to vote on it!
Also here’s the full roadmap, if you haven’t seen it yet: