You could change to 2x2 troffers with 4 lamps each. You could fit in 4 preheat chokes and starters in each and have preheat troffers, which are rare in North America.

Now, you want to use T12, so that will work. However, you can change to European 18W T8 if you want higher CRI, and still use your preheat magnetic system. Such 4x18W preheat systems are very common in Europe. You will need to find lamps in specialty stores or on eBay.

That would work but putting a ballast in those steel pull boxes tends to make them louder than normal unless you really tighten down on the screws and use foam to isolate the cover. The magnetic field from the running ballast tends to make the cover on those vibrate and rattle like crazy. I've have a few homemade ballast boxes made from a steel case have that problem. It's also not that code rated too, most regular core and coil HID ballasts are marked specifically not for recessed use. If you really want to convert an incandescent recessed light to HID I would find some cheap F-Can ballasts from Restore or eBay and use that instead. Much quieter and less janky.

i was just thinking that