Not sure whether you have seen Blackview's BVXXXX series rugged phones - https://www.blackview.hk/smartphone/. I have the BV6000 which I use when bikepacking/hiking. It's pretty much bomb proof and the battery lasts for days as opposed to hours. It's a decent spec (Octa-core 2.0 GHz, 3GB Memory, 4.7" screen) but not amazing. The screen isn't fantastic and the camera is nowhere near as good as they like to make out, but it does the job for navigation. There's also a newer BV6800 with a bigger screen, bigger battery and a bit more memory. Only thing is they seem to have gone up in price lately. When I got mine 2 years ago it was around £140 on Amazon and it seems to be now selling for over £190 and the BV6800 is just over £200, which is still not bad I suppose compared to a lot of phones. They also have a much higher spec BV9900 which is more than double the price but then it does apparently have a thermal imaging camera 
Thanks - interesting. I had come across Blackview but ruled them out as I only saw large screen models. I had been considering risking a second-hand Samsung A3, having discovered the 2017 models are waterproof (Thanks Bolt ^^), but this looks like another good option. A few quick searches show the BV6000 available for <£100.
In case anyone is interested in this, or contemplating switiching to a phone setup, now that I have had good bit of experience with the phone, I'm weighing up how best to configure my setup. As it is, it is great for anything up to, say 600km / 2 days, but I can see that the battery burn rate could be an issue above that - and it is the longest ride that defines the requirement.
There are two main alternative approaches I am considering, and which will dictate what I might buy next:
1) to carry on down the route of looking for the best smaller phone to use as an upgrade to my Cubot and then carry a main phone as well, for back-up navigation and everything else - this is the vision that I started out with.
2) to use my main phone for navigation and use another device as backup. This could be my Cubot or one of my old Garmins, or anything really. When I'm using my phone to do other stuff (find music to listen to, book hotels, etc) I would turn the backup device on purely to show me the route, but this would only be for short periods, so little power requirement. This model has two benefits: I would only have one phone to keep charged, so it
might work out more economical (esp if I upgrade to an OLED screen) and I would have a bigger screen for navigation.
I'm realising that one challenge of taking the phone app approach is that there is a constant temptation to upgrade and fine-tune! And that is not necessarily what I want to spend my time doing.