Been a while, so I don't even remember what I was talking about. I'll answer from first principles without reviewing the thread.
I just joined after a 2 day power outage, and obtaining a HI5 a few months ago. I understand the difference between USA and EU being same voltage but single legged vs double... And being incompatible with shoving into a generator panel input directly... But having 2X the power would definitely be nice. Unfortunately I need 240VAC since that's what my well pump runs, and having the extra wattage is needed since I have 2 fridges and a chest freezer (assuming they all power on simultaneously at some points).
You can't get 2x the power, no matter what, you have 1800W in North America, and that's it, via V2L.
You can get 120/240 with an autotransformer or isolation transformer. The choice between the two depends on worrying about failure modes and costs. However this will not get you past the 1800W limit.
I read through this whole post, and have been thinking of this topic since before acquiring the vehicle. It seems like the best bet might be figuring out how to trick the car into DC charge mode, and then sinking power out instead of forcing it in, then using solar inverters (I saw something about 600V)? Since I live in NW Oregon that's dark for most of the year, and have a lot more timber available than money for solar and batteries... I've spent a lot more time researching woodgas energy generation solutions (definitely dirtier and less convenient than solar) than solar, so I'm a little disadvantaged than the other commenters here.
Unfortunately the E-GMP platform is an 800V car, which means you can't use a standard US solar inverter for residential use. You'll have to import one, which isn't really a big deal as 800V is not uncommon of a system voltage around the world.
Also, there are some code implications. 1- and 2-unit residential is limited to 600V (so it's doubly a code violation -- DIY tapping of an EV is restricted by the latest NPFA fire code, and the 600V limit was in place for a long time).
You can maybe sniff around DCBel to see what they did (though, it's an expensive system, and I'm not sure it's appropriate to ask them pre-sales support about how they handle 800V cars if you don't intend to buy it).
It was definitely annoying to have the generator running for most of the 2 days. I know it wasn't loaded near capacity for much of the time, realistically the V2L probably could have supplied all energy requirements, aside from the well pump. For the peak loading of the fridges/freezer, that would also be a problem, but I guess a small LFP battery, or some big capacitor, would be able to smooth things over during those times. It's quite upsetting that Hyundai hasn't figured this out, as I really like the car over something like the F150 (especially since I already have a reliable gas pickup truck with a little more rural capability/insurance).
Hyundai/KIA group has made a lot of noise about V2X for the EV9 and presumably Ioniq9. I believe they have an inverter / V2X EVSE partner (Can't remember the name)
To get started on sinking DC, I guess use an interposer (can these be purchased, or is DIY needed?) for the charge port and record the low voltage data signalling during a fast charging session?
Unfortunately CCS isn't that easy (what you're thinking of is doable in Chademo and possibly also the Chademo successor that some corners of Japan and China are doing, but that's irrelevant trivia for the Ioniq5)
The DC signaling is on the high voltage conductors. CCS uses PLC signaling on those lines. There is another thread here from a while back, where someone pointed at a dev board that does what you want.
There is also another thread from a person in SEA or Europe, that has a startup making a box that will trick cars into exposing their DC ports. There I think also is a github project, including documentation on the quirks of various vehicles if you bamboozle them.
I don't keep a bibliography unfortunately of these threads or githubs, since I'll never be interested in doing this project. Probably.
And while I vaguely sling around threads, you might wander over to a EV hacking focused forum. This one has a few EV hackers, so it's better than other ones you could have wandered into, but still not ideal.
My current personal plan is very boring. Buy a listed $$$ EG4 system like 18kpv or 12kpv. Get that permitted fully to the $$$ standards that I need for my California POCO. Then add a 120V -> 48V charger onto the DC bus (Chargeverter or Telecom rectifier or Meanwell). This is basically the charging inlet from my Ioniq5 V2L. It will be limited to 1800W, but the buffer battery will be able to surge much higher.
The cheapest instantiation of this would be a 5kWh wall battery and one of those $1500-2000 120/240 inverters. 6000XP, SRNE, Growatt, etc.