Can I set global rendermode "InteractiveServer" and override it to use "StaticServer" per component in a Blazor Web App?

There are two ways to achieve that.

Case 1. To be used if the Routes of all pages that shall use StaticServer start with the same Route-Segment (i.e. are in the same folder).

If for example all Pages in the "Account" Folder shall use StaticServer rendermode

In App.razor add the following

@code {
[CascadingParameter]
private HttpContext HttpContext { get; set; } = default!;

private IComponentRenderMode? RenderModeForPage =>
    HttpContext.Request.Path.StartsWithSegments("/Account")
        ? null
        : InteractiveServer;
}

and set

<Routes @rendermode="RenderModeForPage" />

a render mode of "null" means it will use StaticServer rendermode.

Reference

Case2. To be used if the pages that shall use StaticServer rendering are scattered all over the place.

Create a StaticSsrLayout.razor with the following content

@inherits LayoutComponentBase
@layout MainLayout
@inject NavigationManager NavigationManager

@if (HttpContext is null)
{
    <p>Loading...</p>
}
else
{
    @Body
}

@code {
    [CascadingParameter]
    private HttpContext? HttpContext { get; set; }

    protected override void OnParametersSet()
    {
        if (HttpContext is null)
        {
            NavigationManager.Refresh(forceReload: true);
        }
    }
}

Set this Layout for all pages that shall use StaticServer rendering

@layout StaticSsrLayout

In App.razor add the following

@code {
[CascadingParameter]
private HttpContext HttpContext { get; set; } = default!;

private IComponentRenderMode? RenderModeForPage =>
    HttpContext.GetEndpoint()?.Metadata.GetMetadata<RenderModeAttribute>()?
        .Mode;
}

And set

<Routes @rendermode="RenderModeForPage" />

Reference

When using version 9 and up, if you don't want to rely on a specific path but really want to decide on a page-by-page basis you can use @attribute [ExcludeFromInteractiveRouting] on the static pages.

You will still need the PageRenderMode in the Router as described in the documentation:
Static SSR pages in an interactive app

In regard to what René wrote in their answer. If you want multiple paths for case 1, you can do like this:

     private IComponentRenderMode? GetRenderMode(PathString pathString){
        IComponentRenderMode? StaticServer = null;
        bool isAccountPath = pathString.StartsWithSegments("/account");
        bool isSettingsPath = pathString.StartsWithSegments("/settings");
        if(isAccountPath || isSettingsPath) return StaticServer;
        return InteractiveServer;
     }
    
    private IComponentRenderMode? RenderModeForPage => GetRenderMode(HttpContext.Request.Path);