I can share with you some experiences with two of the companies named. First is Soundsmith, they installed a new saphire cantilever and their top stylus on my Dynavector XX2MKII cartridge. It came back a much better sounding cartridge than it was when new (to my ears).
Second is Expert Stylus and Cartridge Company, in London. I had them modify a Zu Denon 103R cartridge with their white saphire and Paratrace stylus. It is one of the best sounding cartridges I've heard (it compares very favorably with my friends' XV1S/Triplaner combination).
So, I can heartily recommend both these companies. Since you are in Europe, ESCCO is closer. Either way, it will be a three to four month wait, as both have large backlogs.
Andy is in Seattle (west coast, USA), and may not require such a long wait time.
Good luck, regards,
Dan