Okay tried it again after my initial complaints. Manged to beat it on hard with an immortal/archon deathball. Spent solarite on resource income boost first, so quickly got 3 forges+ tech up and running, switched to army-based solarite afterwards. Focussed on teleporting to kill the caves first so I only got 2 dropperlords. Made the game feel a lot easier.
Closing order of caves.
Caves opening on units created is probably too abusable. For example, I purposefully didn't hit the first cap until just before I closed cave 6. On the other hand, caves opening on units created is painful if you lose a lot of units early (zealot heavy strats), setting you back farther. Similarly, players have an incentive to close the stronger caves first.
Some of the caves may need reworking to allow more units to hit them, or so you can put a few immortals off on the side to constantly whittle down hp. Cave 3 in particular.
Bug found: opening both menu's and closing one keeps one menu open but the game unpauses.