It’s pretty hard to get a good way to tell if your about to orgasm without something around your genital/anus area since the reason why the nogasm is a buttplug is so it can be right where your body squeezes before it starts to orgasm. As your penile tubes are on the other side of the anus’s walls along with the prostate. Even then people have reported that the nogasm doesn’t detect well enough or too late, so much so that it’s an known problem I believe with V2 of it which has mean you need to spend some time doing calibrations or something.
Next best is to try and maybe have some setup for the underside of your penis where the tubing can be felt. (I believe behind the balls is the best spot to feel the pulsing starting) However that requires a pressure setup or something to sense the tubes flexing and you can more easily accidentally trick the system from just random flexing or movements you’ll do.
As a result heart rate is the best you can do however you do need find out what is the best way to have it be consistent-ish. (As you said just having woken up your heart rate doesn’t hit 100) Maybe try seeing if doing something prior and then jumping right into this would work like doing jumping jacks or running?