> Why are you using the last one?
The last one is definitely in a different bucket for me than the first four. For starters, all of these are there primarily to encourage conversation. That said, the first four can make a lot more sense to try to graph and track and optimize. The last one tends to be more purely about driving a conversation.
At the top level, if you're genuinely doing a ton of learning along the way to shipping the right feature to the customer, then arguably that value is ultimately reaching the customer. If on the other hand you can't decide what the goal is and you keep changing your mind (as is often the case), then you tend to end up with a lot of dev investment made in things that simply never ship.
It's also worth differentiating technical "spikes" from feature "experiments." In my vocabulary, spikes are things where you're internally assessing a question like "could we do this" and experiments are things where you're externally assessing "do customers want this/does this have the impact we want." If you have a lot of experiments that don't reach customers, you're burning a lot of dev time on things that aren't actually experiments (because by this definition experiments need to reach the customer surface to deliver data). That's a signal you should probably be looking at. Spikes generally only reach the customer indirectly (through an eventual shipping feature), but if you have a lot of spikes that don't ever reach the customer in any way that's also a signal you should probably be looking at.