When you say it’s fun to watch, do you mean it’s only fun to watch and not fun to play?
No! No, not at all. I mean that what's happening on screen is as interesting to spectate as it is to participate in. For example, the reason I picked your game was because the gif you had selected of the swarm attacking the field. That just looked cool and I had to try it.
Thinking about it more, this is closest to a simulation game. The player gives inputs to a system, then the system does the thing and the player gets to watch that unfold. The player just guides the swarm as opposed to having direct control or having a button assigned to 'eat'. Does that make sense? So if you wanted to develop this further, you could look to other simulation games for inspiration. Could you introduce random events like SimCity? Upgrades to move, consume speed, or max swarm size like an incremental/idle game? Stuff like that. As a game dev, nothing should be truly off-limits. Everything has been done, so always be thinking of how you can borrow and modify existing stuff that works well!
Your point about making a spectacle is very interesting, I wonder how much adding more detail like leaves flying away or branches falling would enhance the experience.
I think this is the right line of thinking, but you also need to be wary of adding small effects like leaves. Any small debris added to your game is going to muddy with the swarm of locusts. You could perhaps do yellow flecks that the swarm then eats, but anything else that comes close to being "little bits moving on screen" will only draw attention away from the swarm.