In case this questions is still relevant

Is there a way to set a per-key TTL?

Yes, you can. You need enable this by adding LimitMarkerTTL to kv config while creating kv bucket tho.

Once a KV store has been created, is there a way to change its default TTL, as you can with streams?

Yes, you can change default bucket TTL, existing keys TTL WILL be affected.