Dear Rabbi Simon.
If I don’t make it to shul on a Saturday morning, what is the earliest time I can make kiddush at home?
Technically the earliest is after the Amidah for Shacharit, which can be recited any time after dawn (=72 minutes before sunrise). However it is ideal to wait for sunrise itself for Amidah. If you want/need to eat early, according to some you may do so before Shacharit but without kiddush, as the time for kiddush only begins after Shacharit. Ideally you should recite Musaf before kiddush as well.
I hope this is helpful.
Rabbi Rashi Simon