600W DC-DC 12V to 60V "alternator charger" on Prime 12V batt?

I don't know about the Jackery power converter, but I've done a Bluetti converter unit. That Acc input will only cut-off the unit when the battery/power get to a preset threshold number. In other words, it' on all the time, including pure EV mode. Don't know if you can adjust that threshold cut-off number. I would set it around 12.3VDC-12.5VDC, since the gen4 batteries usually live right around 12VDC. I would also carry a backup jump box - but the car will start all the way down to 10.5VDC; if you pull the fuse to that Jackery converter. That way it doesn't swamp the system as your trying to get it into 'ready mode'.
Make sure you have the properly sized wire for your cable run and properly fuse it. You don't want someone T-boning you and have an active line, ready to spark a fire. Anything over 50VDC can hurt you; 12VDC not so much - just a tingle if your really sensitive. Make sure there's plenty of ventilation around that converter box - DO NOT bury it under your junk; that's a sure-fire way of burning it out.

Good Luck.....

 

The heat pump is driven by the traction battery and the Prius DC-DC converter is good for around 1000W. You'll need to watch whatever you have drawing off the 12VDC system alone. You'll get into trouble if you try to run a 12V to 120VAC inverter at the same time as charging your Jackery; you'll overwhelm the Prius DC-DC converter. Any appliance you can plug into the Jackery would be helpful - your going to use the Jackery's built-in protection system as a safety cut-off - so that 600W charger will charge your Jackery when demand is low. You can put the car into 'ready mode' before you start drawing high current from the Jackery, so you let that unit manage the power demands. It should draw off of it's internal batteries when you exceed 600W, while firing up your coffee pot or electric stove.
I'm actually surprised that an electric blanket draws less than 600W, just warming up then pulses on-off to maintain heat - great for winter car camping, so you don't freeze your butt off.

Good Luck.....