The SMC Pentax A 645 120mm F4 Macro is a great option. More information under
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There are also some sample pictures and user impressions to find.
Furthermore, you can increase the imaging scale by using Pentax 645 auto extension rings greater than 1: 1 or Pentax 645 Helicoid Extension Tube.
Pentax auto extension rings
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Pentax 645 Helicoid Extension Tube
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I use the FA version on the GFX. The version differs from the A version by the autofocus. Since you can not use the AF on the GFX, it is cheaper to buy the A version. To adapt to the GFX I use a Kipon GFX P645 adapter.
Lens, auto extension rings, Helicoid Tube - form a lightweight and versatile macro solution.
Greeting Gerd