The EU knows exactly how the administration feels about them with regards to military support. The Signal thread makes all involved look extremely incompetent. I’m not seeing the advantage if this was planned.

I disagree. When you leak to the press, you often do it with a planted source who "leaks" to a journalist on condition of anonymity. Doing it with an "accidental" group chat add like this signals incompetence without any added value.

Updated Hanlon's razor: Never attribute to intelligence that which is adequately explained by stupidity