Adjusting grease pencil stroke thickness across multiple frames

I had a pretty much done animation and I'm trying scale thickness on selected strokes across multiple frames

What I’m trying to do:

1.  Select specific strokes across multiple frames

2. Adjust their thickness (via Stroke > Set Stroke Thickness or a modifier)

Options I’ve tried:

1. Vertex Group + Thickness Modifier

• I select strokes on specific frames and assign them to a vertex group
• Add a Thickness Modifier and limit it to that group

❌Problem: it still affects strokes in other frames I didn’t select

2. Copy selected strokes to a new layer + Thickness Modifier

• Then add a Thickness Modifier to just that layer

Better control, but I still need to manually delete strokes on frames I don’t want affected

Is there a better way to adjust thickness only on certain strokes across specific frames?