I've got the Nikon Z6II and the Nikkor 8-15. It works very well with the FTZ. No problems.
Add the Kenko 1.4 and make sure you get the right version. I've got two DG and DGX
Nikon Z6III + FTZ + Nikkor 8-15mm + "N-AF 1.4X TELEPLUS PRO 300 DGX Kenko" will not focus or find an aperture
Nikon Z6III + FTZ + Nikkor 8-15mm + "1.4 TELEPLUS PRO 300 KENKO N-AF DG" works fine, will AF and finds the aperture