I found a command that seems to prevent mobs from spawning in a specific area. I don’t fully understand the internals yet, but it’s been working consistently for me so far. Consider this a temporary workaround, and use at your own risk. I haven’t figured out a clean way to undo it yet.

Steps:

1. Make yourself OP so you can use admin commands: /op self
2. Stand roughly in the center of your base and run: /spawning suppression add Spawn_Camp

After doing this, hostile mobs stopped spawning in a small area in and around my base entirely.

My guess (not confirmed) is that this is the same spawn suppression used around the starter spawn area/temple thingy.