Resistance Mode = this is like the old-school type of trainers where you dial up the resistance using a “dial” and there’s a belt that actively presses on the flywheel to make it harder to spin. Resistance mode if not mistaken, is dependent on manufacturers. Programs just sent it stuffs like 10% resistance or 50% resistance. (we don’t send things like watt)
Erg - I think GPLama explains it better in a video Indoor Cycling Smart Trainers: ERG Mode Explained - YouTube
Slope / Simulation Mode - In this mode, what happens is instead of sending the trainer a predefined Target Power (ERG mode), it instead sends it Gradient. Much like when you ride outdoors and the trainer will use those gradients, coupled w/ things like wind / drafting factor / tyre resistance / gravity / user weight and then the Trainer (via it’s firmware) will translate that into a set of wattage needed for the user to ride such that it’s “like” riding outdoors. Simplest example of Slope Mode is Zwift. Here’s another example of how riding a slope mode workout would look like.
YOu can create a workout in Slope Mode using TrainerDay’s workout Creator (note that a subscription is needed) - note the red box
you can also read more about it here -Follow a route - TrainerDay
This is what coros has to say. Have you tried it? I do not have a Pace 2 and my Garmin doesn’t Broadcast HR
https://support.coros.com/hc/en-us/articles/360040256991-Broadcast-heart-rate