I currently have a nested recursive component calledDbVariableGrid as follows, keeping minimal snippet:

<MudDataGrid ref="@DataGrid"             T="DbVariable"             Items="@Items"             Groupable="true" GroupExpanded="false">    <Columns>        <HierarchyColumn T="DbVariable"/>        <PropertyColumn Property="x => x.Name"/>            @if (Items.Count == 0 || Items.Any(i => i.GroupIdentifier != null))        {            <PropertyColumn Property="x => x.GroupIdentifier" Hidden="true"                                           Grouping GroupBy="@(x => x.GroupIdentifier ?? string.Empty)"/>        }    </Columns>    <ChildRowContent>        @if (context.Item.Items.Count > 0)        {            <DbVariableGrid Items="@context.Item.Items" />        }    </ChildRowContent></MudDataGrid>

Currently, I avoid the PropertyColumn, if no item has GroupIdentifier in the given depth and generate the next layer if there are more Items of an item of this layer.

using Row Detail View (Hierarchy Columns).
adjust your data
have each RowDetail have children and create multiple groups within

I'm not sure I get it right.
Are you suggesting to drop the basic Property-based grouping and implement the DataGrid against some List<Group> instead of the List itself? Then another List within the ChildRowContent?

So basically something like this?

<MudDataGrid T="Group<DbVariable>" Items="@GetGroupedList(Items)">    <Columns>        <HierarchyColumn T="Group<DbVariable>" />        <PropertyColumn Title="Group" Property="g => g.Key" />    </Columns>    <ChildRowContent>        <MudDataGrid T="DbVariable" Items="@context.Item.Items" Groupable="false">            <Columns>                <HierarchyColumn T="TItem" />                <PropertyColumn Property="x => x.Name" />                <PropertyColumn Property="x => x.GroupIdentifier" Hidden="true"                                              Grouping GroupBy="@(x => x.GroupIdentifier ?? string.Empty)"/>            </Columns>            <ChildRowContent>                @if (context.Item.Items.Count > 0)                {                    <GroupedDbVariableGrid Items="@context.Item.Items" />                }            </ChildRowContent>        </MudDataGrid>    </ChildRowContent></MudDataGrid>

Apart from extra styling, hiding headers, it still feels quite some effort to apply uniform column widths this way, but I'm really not sure if this is what you meant. Did you have something vastly different in mind?

I was suggesting a 1 level hierarchy that handles the initial grouping, and then any additional grouping can be handled inside the rowdetailview. I did forget about the column alignment woes but I usually add some padding around the childrowcontent and leave the header of the child grid so I don't have to worry about it. That's a preference and yes your last example looks pretty close to what I was thinking.

So your groupeddbvariable grid likely won't have the same woes and can just do regular grouping. If you have more than 1 then using hierarchy column that needs the special grouping you would almost have to be aligned or look odd.

Given theInitiallyExpandedFunc andButtonDisabledFunc to a Hierarchy always-open programmatically, I am now in the belief the same concept would be a neat way on theGrouping as well.

I haven't checked the code behind, so I might be wrong, but someGroupingExpandedFunc would be quite awesome.

Unfortunately, theGroupExpanded is shared, so that wouldn't really work now.
Would you see handling such function for Expansion (and maybe ButtonDisabled) still too high effort?

No, that effort level would be probably be in line and acceptable. However, you could probably do this a bit more manually using GroupKeyPath with the public DataGrid method ToggleGroupExpand.

https://mudblazor.com/api/datagrid#methods

/// <summary>/// Represents a read-only, ordered collection of group key values forming a unique path through nested group levels./// Used to identify the exact group or subgroup location in multi-level group scenarios./// </summary>/// <remarks>/// Two <see cref="GroupKeyPath"/> instances are equal if they contain the same elements in the same order./// </remarks>publicclassGroupKeyPath(IList<object?>list):ReadOnlyCollection<object?>(list)

Now I did not implement the GroupKeyPath, someone else did but the code is likely available now to either implement an InitiallyExpandedFunc or something do it manually..