By all means, buy an android device if that tickles your pickle. But low internal space =/= Microsofts fault. The problem you are having is that your phones are out of date. If you were android users, you'd also be running out of space. Phones ship with between 16gb and 128gb now.
Personally I think it makes sense, not running a process that could result in a bricked device if failed, via a removeable storage medium. I seriously doubt any mobile OS operates that way.
I'm also not surprised it finds it hard to run on 8gb. It might be nice if you could flash the OS from a PC though, at least that's not something casual users would try, so it would be on you if it failed. However the issue there would be drivers - you would need a current updated full image, and who's going to be making that for old phones? Sure as heck not android OEMS either (a lot of android phones don't get updates on OS, a year after they are released. If you get them two years after, you count yourself very lucky).
With my 8gb storage bb10 device, I had to hard rest to install the latest update. It didn't even have features, lol, it was just a security update, and my phone couldn't handle it. These problems are not unique to you, or windows 10 mobile. I suggest if you want to try and make your current machine work for awhile longer, to do a hard reset. After the upgrade is complete, it should use less space than when installing.
Although it strikes me as odd, that windows 10 mobile users complain about not getting OS updates 3-5 years after there phone is released.
If you buy an android 6 phone right now, odds are, you will never get to see seven. Android users to a little dance of joy if they are amongst the elite who get offered an upgrade. It's actually considered 'newsworthy' when a model gets an upgrade.
Must be interesting having grown so comfortable with ongoing updates and upgrades that it becomes an expectation.
Also, yeah, low storage will cause issues with speed, because all operating systems run a disk cache.