Trying out fire... got a few questions

Trying out fire for the first time, and I have a few questions about rotation and ability usage.

-If I’m running Searing Touch, how do I approach the execute phase? All I’ve seen suggested is “replace Fireball with Scorch”. Do I still use Fire Blast when Heating Up procs, and do I still try to use an instant Pyroblast when Scorch finishes casting? While I’ve been getting the hang of the standard rotation with Fireball, swapping to Scorch seems to throw something off, and I’m not sure what it is.

-Do I ever cast Flamestrike during Combustion? Mostly wondering for M+ and how to appropriately use Combustion during trash pulls. (Is the real solution to just run frost for M+ though)?)

-My mage has a Shiver Venom Relic. Is there anything I need to be aware of for using it as fire? I assume it should not be used during Combustion.