Oven does not have a temperature notification sound
Oven temperatures vary depending on insulation and, the big, how often you open the door. I would recommend using the described 'remote' thermometer or an oven thermometer positioned where it can be read through the glass in the door (assuming that would be possible). Set you oven to it's highest setting and time how long it takes to reach that temperature. Once you know how long it takes to reach 'max' use a timer that will give you the audible cue you are seeking at that length of time, regardless of what temperature you may be using at that time. If you want your oven a 325°F and it takes 10 minutes to get to 500°F...it will 'safely' be at 325°F in 10 minutes, once you test it, trust it. Baking rarely requires such an exact temperature control that the variance created would be problematic. 

Extending the pre-heat time for your oven is not a bad thing, indeed it helps with the recovery time that it takes to return to the set temp after being opened and closed.