bit of code to add Mode Buttons to the Modes Toolbar in UE5 (to the right of Modes dropdown)

gistfile1.txt

	// grab some global stuff from the level editor
	FLevelEditorModule& LevelEditorModule = FModuleManager::GetModuleChecked<FLevelEditorModule>("LevelEditor");
	TSharedRef<FUICommandList> CommandBindings = LevelEditorModule.GetGlobalLevelEditorActions();
	const FLevelEditorModesCommands& ModesCommands = LevelEditorModule.GetLevelEditorModesCommands();

	FToolMenuOwnerScoped OwnerScoped(this);		// not sure what this is for exactly, but other UToolMenu extenders do it
	// hardcoded name for the modes toolbar, from FLevelEditorToolBar::RegisterLevelEditorToolBar()
	UToolMenu* ModesToolbar = UToolMenus::Get()->ExtendMenu("LevelEditor.LevelEditorToolBar.ModesToolBar");

	// add a new section
	FToolMenuSection& NewSection = ModesToolbar->AddSection("MyModeButtons", LOCTEXT("MyModeButtons", "MyModeButtons"));

	TArray<FName> IncludeModeIDButtons;
	IncludeModeIDButtons.Add(FBuiltinEditorModes::EM_Default);
	IncludeModeIDButtons.Add(FBuiltinEditorModes::EM_Landscape);
	IncludeModeIDButtons.Add(FBuiltinEditorModes::EM_Foliage);
	IncludeModeIDButtons.Add(TEXT("MeshPaintMode"));
	IncludeModeIDButtons.Add(TEXT("EM_ModelingToolsEditorMode"));
	for (FName ModeID : IncludeModeIDButtons)
	{
		// this is how the command names are constructed in FLevelEditorModesCommands::RegisterCommands()
		FName EditorModeCommandName = FName(*(FString("EditorMode.") + ModeID.ToString()));
		TSharedPtr<FUICommandInfo> EditorModeCommand =
			FInputBindingManager::Get().FindCommandInContext(ModesCommands.GetContextName(), EditorModeCommandName);
		if (EditorModeCommand.IsValid() == false) 
			continue;
		// cannot directly add the FUICommandInfo here because it is not a Button type and InitToolbarButton will ensure :(
		NewSection.AddEntry(FToolMenuEntry::InitToolBarButton(
			EditorModeCommandName,
			FToolUIActionChoice(EditorModeCommand, *CommandBindings),
			EditorModeCommand->GetLabel(), EditorModeCommand->GetDescription(), EditorModeCommand->GetIcon()));
	}
	NewSection.AddSeparator(NAME_None);

you will need at least these headers:

#include "LevelEditor.h"
#include "LevelEditorModesActions.h"
#include "ILevelEditor.h"
#include "SLevelViewport.h"
#include "ToolMenus.h"
#include "EditorModes.h"

and the "LevelEditor" and "ToolMenus" modules in your build.cs (and probably others)

Running this when the FEditorDelegates::OnEditorInitialized() delegate fires might work (or possibly in a module init)