Okay, sorry it was my mistake. I missed to use D3DImm.dll in combination with ddraw.dll and D3D9.dll.
Now everything works perfectly. Even the fps drops are gone I mentioned in this topic in 2019. Everything runs smooth.
But for some reason now you notice fps drops, Dege?

But some questions remain:
This is a dx6 game so normally D3D8.dll (for dx1-8 games) would be the right file to use. But why only D3D9.dll works while it's for dx9 games only?

And there is still the strange window behavior when dying in the game, finishing a level or open a menu screen.

Well,
- either you overwrite GOG's ddraw.dll with dgVoodoo's ddraw.dll + d3dimm.dll
- or use GOG's ddraw.dll combined with dgVoodoo's d3d9.dll

to run this game. In the first case the game is wrapped from DX6->DX11/12 directly. In the second one, the game wrapping chain is DX6->DX9->DX11/12.
Last time I only tried the first case, and indeed, I had only stuttering ~45-50 FPS's. It must be some timing issue (according to a quick profiling).

With window behaviour, you mean the game window gets minimalized when the load/save dialog box pops up?

Dege wrote on 2021-08-06, 13:43:

With window behaviour, you mean the game window gets minimalized when the load/save dialog box pops up?

Yes, for example. This also happens when the player dies, finish a level, or everytime when a dialog box pops up. The game window gets minimalized and some times fails to get back full screen. Maybe you can fix this?

Also some bitmap graphics are missing in the dialog boxes (I could fix this using the DisableThemes & DisableThemeMenus appfix from MS compatibility toolkit).

Yes, it's minimalized because the game render window loses the focus. Disabling option 'Disable Alt-Enter...' + enabling Fake fullscreen rendering should help but I didn't try it.

By default, even if dgVoodoo didn't minimalize the window, DXGI would always push the game window out of fullscreen state because of the dialog box.