Update:
Tried the usually recommended 10% sodium carbonate solution for 10 minutes to 'develop' the plates, but the E9220 ended up being extremely brittle and flaked off easily.
Eternal's technical data sheet for E9220 recommends a 'development' time of 60 to 70 seconds in a 0.85% sodium carbonate solution at 30C. That might be fine in a massive industrial machine, but hardly doable for hand washing out polymer photogravure plates.
A first stab in the dark guesstimate for a useable washout procedure was to try 5% sodium carbonate at 65 degrees F (basement in winter darkroom) for 3 minutes while using a hake brush after 30 seconds. Dried it via blotting and 5 minutes with a hairdryer on high, then back under the UV light (fifteen 24" Barrina UV LED light bars on about 1.25" centres) for 2 minutes.
This resulted in a coated plate where the photopolymer doesn't chip easily...you really have to dig at it with a fingernail for it to lift, even at the corners.
To strip the photopolymer film from reusable PETG plates, I tried isopropyl alcohol but it didn't work well at all, and it stinks.
I don't have dry sodium hydroxide, so found a grocery store solution. I read that drain cleaners/openers are somewhere around a 50% solution of sodium hydroxide. Found a budget liquid drain opener that contained sodium hydroxide and a small amount of surfactant.
Slowly added 100ml of the drain opener to 900ml of water (a ballpark 5%-ish solution) and it completely cleared a plate in a bit over 20 minutes. Added another 100ml and the photopolymer completely lifted off in under 15 minutes.
Anyways...just sharing some findings from a complete newbie clawing up the learning curve