> Just plug several units into a power strip ¯\_(ツ)_/¯.

Hehe, ok. I think that's your insurance company calling on the other line.

> are you exporting power

Exporting 12 MWh / year or thereabouts.

> or zero export with a current monitor upstream of the main panel?

There is a current monitor (a Shelly tri-phase one), right now it is still economically viable to do so (though the utility companies are trying what they can to dissuade you by changing the deal through politics). If it is no longer then I will just install a battery and disconnect from the grid for the summer months.

And I'm not using a transfer switch because I don't have a battery to stabilize the system.

Well, technically those breakers are a manual transfer switch, only it is broken up into two halves and I can just disconnect the mains feed and run in island mode but I would still need to install a battery and a charger. House mains breaker off, solar on would be the house running entirely off the local stuff, I just don't trust that inverter without a battery behind it to be able to react quickly enough to load changes and by default it is set up to disconnect if the grid goes off, so you'd have to manually override that. The main issue with it being two halves is that you can not guarantee that the house net is in-sync with the grid at the moment you make the switch and that's a bad idea with a system this powerful, so I'd definitely get a proper automated one if I intended to do this for real, otherwise you might cause a load spike which could trip breakers and annoy the neighbors.

Right now I can't switch that on or off under load anyway because the large inverter would simply disconnect as well.

Tri phase breaker of the right amperage was about 150 bucks.

If I were to do this I would probably get a complete set from Victron, their stuff is amazingly well engineered, but if these open source people are going to make an inverter/charger combo then I might go for that and add a another manufacturers automated transfer switch.

An inverter is, complexity wise, not that much harder than a large switching power supply, there is some more instrumentation and some more rules but it isn't super difficult. It is much harder to make one that is commercially viable because those guys all cut corners to stay competitive. Ironically a proper case is probably the hardest part, there are also some larger inductors that might be tricky to source. And if you were to design one you should probably make the low voltage stuff (UI, CPU) on a completely separate board from the line voltage stuff and go for tri-phase right away because it is so much cleaner. Bonus points for modularity of the output stage.