Regarding the subscription: this would be a huge turn off for me. I guess I disagree with the price model, so I'd only get into a recurring cost for something I deemed essential, whereas I'd be much more likely to pay a (similarly?) small sum as a once off. This isn't criticism, just opening the possibility that sales might be a lot higher as a once off, and if you end up rewriting/adding a tonne of features you could always release V2. I'd be curious how many years an average paying user would actually end up using/paying for with a tool like this.

Obviously this is purely anecdotal, and this extension isn't applicable to me personally anyway.