Good points, as ever.
TATP is simple to make and the precursors are easily found, which clearly offsets the risk of (trying to be) getting more potent explosives for a bunch of first-time terrorists. Fortunately, ISIS has so little regard for individual operatives or, indeed, groups, that they are unlikely ever to have a chance of learning from experience.
The reveal earlier today that ISIS were taking hostages’ mobile phones and attempting to use them for covert internet access is an example of them getting smarter, however. They are clearly using the Internet as the ISIS communal “knowledge store”. As such they are getting slowly smarter.