Hi everyone! So it's been a number of months since I asked this question, and I'd like to thank all of you for your comments and suggestions!
I have an update. I decided to stick with my D500, and purchased an Ikelite housing with a dome port for the fisheye. The primary reason for this choice was weight, the secondary one was that I already have an Ikelite housing for my RX100, and was already quite happy with the first one! Finally, the point that @TimG made about saving the money for the next trip was a great one 🙂
I included the vacuum pump option with the housing, and have felt remarkably confident with it, to the point that I upgraded the RX100 housing to also include one.
Pros: Lighter; cheaper; service has been quick (one of the latches was a little bit off-centre and was difficult to open -- ie it didn't spring open like it should, but the clamp definitely held once on!)
Cons: Inserting the fisheye into the dome port requires a bit of practise, as you are almost doing it blind, and then while holding the camera in place, flipping it over to check that that the 'tongue' has entered the groove of the zoom gear ring; being acrylic, I am concerned about scratching of the dome port and the back plate.
Welcome any questions and/or comments.
Happy diving!