Tapping off of service entrance (load side)

Hello. My existing service entrance comes into my attached garage to a panel that contains only an OCPD/"master switch" (please excuse my terminology - I'm not an electrician but I think I have the right terms here). From there, service goes into the basement to what is the main panel. I'm interested in putting a panel in the garage essentially right next to this service entrance panel/master switch. I am putting in a 240v heater and a table saw and will be running a small workshop from time to time in the garage. I want to run 100 amp service to a panel there in the garage and then provide a dedicated circuit for the heater (a King Electric unit) and then probably two outlet circuits (120v, 20 amp) for machines like the table saw. My question is how do I go about tapping off of the panel that's in the garage rather then running all the way back to the garage from the current main panel. And, related, can I just tap off the existing garage panel (and leave the current wiring completely alone), or do I have to put this new panel "inline"? I'm attaching a photo of the service disconnect panel that is in the garage. Any advice on how best to get a run off of what's here to a new panel I'll install like 2 feet away (say on the side of the same stud) would be most appreciated. NOTE: there is definitely not any cable slack with what's already there, so if there is going to be any splicing required, please provide details on how I do that and with what devices.