See Outline: Fold to specific heading level (consider merging)
At the end ofthe thread, someone suggests a plugin Quiet Outline, in particular this feature “Default level”: GitHub - guopenghui/obsidian-quiet-outline: Improving experience of outline in Obsidian > Usage: Release 0.3.33 · guopenghui/obsidian-quiet-outline · GitHub which I tried myself and it’s working well.