QGIS outer glow effect without self-reinforcement

You probably use the wrong "Draw Effects" button. There are two "Draw Effects" options.

You should use the one under "Layer Rendering".

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You may want to review this post: Why are there two Draw Effects buttons in Layer Styling Panel in QGIS?

Choose your glow in Render-Settings for your layer, so it renders for the whole layer, not for each geometry:

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For the example below I am using these settings:

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As you can see:

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