[Android Q&A, Help & Troubleshooting] snake2332:
For anyone interested, here is what happens when I start and stop my car:

Turn key on -> Power inverter under my seat gets power and sends juice to 3 USB devices: Dashcam, Nexus7, and Note4

Galaxy S9 (primary phone, usually in my pocket) connects to Sony head unit, with bluetooth settings set to phone audio only (no media audio so the Sony head unit allows a 2nd BT connection from Note4 for media).

Note4 and Nexus7 boot up from powered off if car was off long enough. Nexus7 takes around 1 minute to boot up, the Note4 with CyanogenMod and no Xposed FW takes ~90 seconds to boot up, so we sit in silence while we wait. Note4 might be capable of staying powered on all day, with a shutdown scheduled at like 7pm so I only have to wait for boot-up once a day in the morning. It would be shut down for half the day anyway.

Note4 - Tasker detects STATE = POWER AC and changes Power Mode to normal, enables bluetooth, disables wifi in case I had turned it on manually, enables wifi hotspot for Nexus 7 to connect to, forces bluetooth connection to Sony head unit a couple times (to help it reliably connect), launches SiriusXM, waits, attempts Media Control - Play, waits longer in case SiriusXM app is opening fresh and takes 10+ seconds, attempts Media Control - Play again, launch Automate

Nexus 7 - Tasker detects STATE = POWER AC and launches Real Dash. Real Dash is set to enable bluetooth if it's off and connects to my OBD-II adapter. I don't have the Nexus 7 doing anything else at the time.

Turn key off -> Power inverter no longer has power

Note4 has some Tasker profile state "exit tasks" that it performs to help save power, like power mode = battery saver, bluetooth = off, kill app Automate + SiriusXM because SiriusXM doesn't work reliably when music is paused and data connection is lost or phone enter deep sleep, etc., then it runs the app Screen Off to turn the screen off immediately (just so I can be assured the exit tasks ran when I'm exiting the car and I see the screen turn off).

Nexus7 has no exit tasks.

Note4 has a half-hour countdown before it turns off via the app Power-off Shutdown. This gives me enough time to run into a store and get a few things and my music will resume quickly when I get back into the car. I will probably increase this to 1 hour for those longer in-store trips, as long as the Note4 still has enough battery juice to do a full hour all the time. Or, like I mentioned before, I'll disable this app completely and will just have a scheduled shutdown at 7pm every night so the Note4 is powered off for half the day. Hoping this is possible without the addition of a battery pack of sorts.

Nexus7 has a short 2 minute countdown timer before it shuts down. This delay is necessary because you don't want the devices to shut down as SOON as power is cut, because this will occur when you go from ignition position 1 (like listening to tunes while getting fuel) to position 2/3 (car start / on). I was thinking of keeping it at half hour or full hour like the Note4, but the Nexus7 battery charges quite slowly even at 2.4amp and I think it will slowly drain faster than it will charge over time.

I generally drive my car about 1 hour a day, Monday thru Friday. I need the devices to gain enough battery charge in that amount of time to power themselves. The Note4 seems to gain a decent amount on my half-hour drive to work, like it goes from 88% to 97% charge. But my Nexus 7 usually only gains a few battery %, especially when it's closer to 95%+ where it takes more energy to charge. Luckily, the Nexus 7 is only really used for Real Dash and big navigation maps, so I don't mind waiting for the boot-up sequence every time I get in the car. I have a 10,000mah battery pack to refresh the devices if they get low, rather than leaving my car on for a long time to charge the devices (namely the Nexus7 because it's perma-mounted in the dash). Over time, I will fine-tune this config and see what works best. The Note4 seems to be using very little power when it's dozing, so it might just have some scheduled shutdown times. I wish I could schedule a boot-up time, hehe, but really I start my car and let it warm up a little at least 6 months out of the year (Montana has cold mornings more than half of the year), so the boot-up time in the morning doesn't bother me. In fact, I will set up the Note4 to do scheduled shutdowns right now and see how it works, long term.

Anyway, enough blabbering about my config. I'm just happy that it's kinda working, finally. Now if only I can figure out a way to keep SiriusXM from going into double-speed playback. Maybe if I reduce the playback quality from max to high, but I really don't want to do that.