お疲れさまでした。気になってみてみたのですが、次の文はBeyondの基本ルールからですが、
Level 2: Reckless Attack
You can throw aside all concern for defense to attack with increased ferocity. When you make your first attack roll on your turn, you can decide to attack recklessly. Doing so gives you Advantage on attack rolls using Strength until the start of your next turn, but attack rolls against you have Advantage during that time.
次の自ターンの始まりまで筋力攻撃ロールに有利を得られるなので、宣言中の機会攻撃は有利なんじゃないかなと思いました。賢者アドバイスでは特に触れられていませんでした。
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