Synthesia is fairly RAM-efficient, usually running at less than 100 MB unless you load a song with millions of notes. There isn't anything on the roadmap that should change that very drastically.
That 4.4 version of Android is the more worrying part. According to the
Android Dashboard, version 4.4 is already down to 7% of active devices. Normally we take <1% as a cue that we can finally stop supporting a legacy version of an OS, but I've recently been mulling over making a bit of an exception for Android. We've got a
very small development team and Android seems to cause at least 75% of our support/bug-fixing workload despite being responsible for a much smaller fraction of our revenue. After recently stumbling across Paul Graham's
Life is Short essay, I'm beginning to wonder if life might not be a bit too short to continue chasing down weird bugs on old, unsupported Android hardware.
I haven't made any decisions yet and I'm not sure what one might look like, but I expect that it will involve bumping the minimum OS requirements on Android more aggressively than the other platforms. We'll see.
For at least a few years at any rate, the Android 5.1 model they've got there in your Alibaba link should be just fine. A 32" touch screen actually sounds really cool.
My setup topped out at 27" and that is kind of a ridiculous price for a 32" screen (no matter the resolution). I am tempted to check it out myself.
