Dissocatives (nmda-antagonists) and benzos don't mix very well imo, as the benzos are going make the ketamine experience harder to remember as well as reduce the desired psychedelic effects. When ketamine is used in anesthesia, a benzo hypnotic like midazolam is used to limit the occurrence of psychotropic effects that are reported when reemerging from anesthesia, and being induced. NMDA-antagonist already interfere with the bodies ability to form and retain new memories, and GABA-A modulation via benzodiazepines also has negative effects on ones memory. The combination of the two is seen commonly to diminish the "psychedelic" effects desired, but increase the sedation as well as the potential memory loss/amnesia. Also, ketamine does not have any significant respiratory depression at all, especially at recreational doses. Ketamine is used in some emergency surgery's that need special care to not cause any extra respiratory depression, because of this ability at anesthetic doses. The feeling of respiratory depression from ketamine is I 99% guarantee you to be in your head.
150mg of ketamine is a decent dose, enough to give you a medium to medium high intensity experience, so I put my money on the xanax reducing the desire effects you are looking for. Generally ketamine isn't as commonly cut as other street drugs due to it's large availability from over seas as pure ketamine. Majority of it will be pretty much pure, so dealers are less likely to cut. I'd try a lower dose with out the xanax to see if you are comfortable with the ketamine alone. If you are, go for a higher dose. A lot of individuals find it to be anxiolytic unlike psychedelics like LSD or mushrooms that can easily cause anxiety, so maybe you will have no issue.
Also, I think the no mixing with other depressants with ketamine is kinda BS. Barbiturates you need to becareful because of the low theraputic index associated with barbiturates, but combining with benzos aren't going to cause any respiratory depression to be concerned of. This is commonly done at doses for both drugs higher than what would be used for recreation. Opioids wouldn't be an issue really either unless you are already taking such high doses, or extremely potent opioids (fent or analogues), which would cause respiratory depression with-out ketamine in the mix. If there are already some respiratory depression going on with another drug, there is a very, very small chance of ketamine increasing it, but you should be worried about the fact that you already have respiratory depression happening...