How can I set a fixed limit (threshold) line to a bar graph? (Not a trendline). I want to monitor anything that exceeds the limit/threshold.

Define a color that will persist when you do a runtime filter or just drill down: for e.g. 2023 = green bars and 2024 = blue bars. When you drill into 2023 data then it should retain the same color (green) in the next level down.