I was going to sign up to a subscription but I'm confused.

I'm only going to play on Android so is the Lite subscription of no use to me? It looks like it doesn't do anything other than allow me to play on a PC which I'm not going to do.

Is the idea that you don't subscribe on Android and just buy crystals instead?

If you want full access to the game then you will need to subscribe. If you don't mind having restrictions or are not motivated to level past license level 4, then the free to play model is fine.

So does the Lite subscription do anything other than allow access to the free-to-play model from a PC? Does it grant you a higher level cap than standard free-to-play for instance?

No, it only allows access from a PC.