I also loved my Z4M roadster very deeply and even so; I also sold it after 8 years of ownership (I never thought this would happen). I have a M2 since the end of 2016 (daily driver). I have kept them together for two years (M2 daily / Z4M sun & fun). But at the end of 2018 I came to the conclusion I was not driving the Z4M that much any more and it was just money standing there. It was a shame to keep the car sitting in the garage most of the time and (for me) cars need to be driven, so the money I spent on the car has a actual purpose. I sold it after the summer!
Two reasons; I wanted to have something not BMW M related as a future second car instead of two BMW M's and for the time being I could use the extra money for some nice investments. Honestly; it's now sold for more then 5 months...and I did not miss it that much. But and this is a
"big" but; the main reason I don't miss it that much is because I love the M2 so much. Only when the sun is really shining I miss driving with the roof down. I miss the sweet high revving NA engine and better steering feel also, but not that much as I would think before selling it. Driving topless was so fun to do on sunny days...but my M2 is so much fun all day...every day (I did not expect I would love the M2 this much). The money I got from selling the Z4M also does good things for my future and this gives me a nice and happy feeling also. So in the end; I have absolutely zero regrets of buying, driving, modding and selling my Z4M
By the way; my Z4M was a rather high mileage car (100.000 miles) because I drove it a lot and never cared about future value. I tried selling it with the AP brakes and all the modifications but this did not work very well and took for ages. In the end I sold the parts (even the hardtop) all separately and this way I got the best deal and the most money on the car. It was one of the best fun cars I could ever buy that was also a good financial deal when it came to buying and selling (never expected this when buying it in 2010).
Good luck with the potential sale....and after I bought my Z4M back in the days; your advise on buying the AP brakes for ultimate track fun was absolute 100% correct advise!