Pop-up flash diffuser

Hi, I'm interested in getting into macro photography. To experiment a little bit I thought I would use my 50mm 1.8 lens on my aps-c sensor R10 body and use the built-in flash with a diffuser. Does anyone have any recommendations for a diffuser?

If I'm interested in pursuing macro further what would be the next logical step? The 85mm f/2 macro lens with a Godox MR12 Twin Macro Flash set-up? Why is the 85mm f/2 so much cheaper than the 100mm f/2.8?

I recently bought the 1.4x extender but I'm considering returning it and putting that money towards the macro lens or a flash. I bought it for my 100-400 lens but I'm not sure the extra zoom is worth the loss of light and 400mm was already pretty good on its own since it acts as 640mm on my R10. I still think there might be some situations where it is useful though like astrophotography (896mm equivalent). And f11 is not THAT bad for that kind of reach, is it?

I should also mention that I'm using macro close-up filters on my 50mm for macro (1x, 2x, 4x and 10x).