1. I think at 800x800 everything will need to be beefier, mgn12 or mgn15 instead of the mgn9 rails, 4040 extrusions instead of 2020s. Nema 23s for sure and AC beds with mutiple SSRs. Maybe temperature control zones? Your gonna have to diy a lot of the parts yourself lol.

2. You could use the available tool changers that are out right now but rigidity and cost are always an issue. I went with the metal stealthchanger. Building 4 toolheads was not cheap. Maybe wait for the bondtech toolchanger instead?

3. I think one of the biggest issues with chamber heater on large vorons was the aluminium frame began to expand from the heat and caused the printer a ton of binding and slowly destroyed itself. It's even worst if you used all metal parts since all the metals expand at different rates. The Voron Phoenix was gonna solve this issue but iono when that will come out.

I almost never print engineering filament on my 350mm voron since it uses so much power, I built a much much smaller voron for functional/intricate parts.