I think you may be analyzing a bit too much. Before I started working on my heeltoeing, I practiced rev-match downshifts and then braking.
Here's a scenario. Let's say you are cruising in 5th gear doing 2-2.5k rpms at about 45ishmph and you see the light ahead of you change yellow from green. As soon as you see the light change, you downshift to fourth by rev-matching your car trying to match the engine rotation speed to the transmission rotation speed. In other words, if you know your 5th gear 45ish mph engine speed is 2.5k rpms and you know your 4th gear is 3.25k rpms, you blip the throttle to increase it to that speed and let the clutch out. If there is no jolt or you see your rpms not being forced upwards or downwards anymore then you did it perfectly.) Once your shifting is complete, brake until your rpms drop some more and rev-match again to 3rd gear.
In my opinion, the perfect training scenario for you is to find a straight road with no traffic around that has a speed limit of about 45mph. Your mission, should you choose to accept it(sorry I had to lol) is to hold 45mph in 6th gear and downshift into 5th, then 4th, and then 3rd while briefly trying to maintain 45mph in each gear before downshifting into the next gear. This will help teach you the rpm difference from gear to gear.
Once you master this, try skipping a gear (go from 6th to 4th or 3rd). This will require a larger throttle blip. DO NOT try to "man-handle" the shifter and try to shift really quickly. You may accidentally mis-shift. The safest way to go from 6th or 5th gear into 4th or 3rd is to let the shifter slide into dead-center neutral and pull forward or backward for 3rd or 4th gear.
Once you have mastered the rev-match, continue your skill building with heeltoeing.