I think it's worth to implement a queue similar to other games, where items can easily be reordered.

It takes quite a lot time for managing the queue especially in late game, due to its inconvenience.

The main pain point, apart of lack reordering, is inability to cancel the first item.

Also, it would much simplify the game if multiple items can be bought, not only the first one.

You can "cancel" the first item by left clicking on the ship you would like to build
It does not exactly qualify as cancel as it will not effect the queued items (second will not become first automatically) but on the plus side, progress on the ship will also not be lost

Buying multiple items is probably a balance decision: it prevents any race from instantly rushing a whole fleet into existence though admittedly Metal shortage does take care of this Still if implemented, it would result into some big fleets popping up quickly: I think other 4X games also do not allow this for the time being. Is also a micromanagement risk as the player would be tempted to micro rush a fleet on various planets - whereas the effort is currently divided between multiple turns when other more interesting activities also occur