Is there a way to define a list of custom properties for all tiles in a tileset, instead of one by one?

This is currently not possible, but it is a planned feature. See the following GitHub issue:

The default will be space-conserving. I could consider to add an option for “verbose” export, it has been requested before.

Note that right now, you can get the verbose behavior by selecting all tiles in the tileset and then adding your property. This would need to be done for each tileset, but that’s a lot quicker than doing it per tile.

That’s an interesting idea! Could be implemented once the above GitHub issue is done.