I'm also a big fan of index cards for planning. I just took another first-time entrepreneur through a planning session and it went very well. My starting advice here:
http://williampietri.com/writing/2014/simple-planning-for-st...
And here's how we used them to run a startup for ~2 years:
http://williampietri.com/writing/2015/the-big-board/
You're not alone in being bothered by finality. If I sense any of that, one of the first things I have people do is test their pen on a card. Then I have them tear up that card. Then, later, I'll have them write an obviously too-big planning item, like "world domination". We then tear that up and break it down into smaller cards.
With a little practice, people quickly learn that cards are temporary, just tokens that represent conversations. And they also learn that reorganizing their thoughts is good and beneficial, a sign of progress. And personally, I think it way, way easier to reorganize a stack of index cards than anything in a computer.
One of the especially interesting things to me is that computers do make certain things easier, but they're often things that I'm reluctant to make easier. E.g., computers can store effectively infinite amounts of stuff. I once saw a ~10-person team with a bug database that had thousands of open items. A database like that is way worse than index cards. It was just a big soup of sadness; people struggled to find ways to stay focused on what really mattered. Eventually a new product manager had to declare ticket bankruptcy and start fresh.
With index cards, requiring manual manipulation and physical space forces people to be more practical, more reasonable, more aware of what's going on. We can only keep so many things in mind, and we can really work on a much smaller number of things than that. Matching those human limits to physical limits in planning tools can produce much better results.