Seconding kit Vorons. While they certainly aren’t the cheapest option, they aren’t that much more expensive than other higher end core XY machines. Fabreeko* has the 250/300mm Trident kit at 1.2k and 350mm 2.4 at 1.4k + an extra $150 or so for printed parts. Compared to the new Core One at between 950 - 1.2k or the X1C also at 1.2k you’re looking at an extra $150 to 350, but that also comes with build volume increases (both the Prusa and Bambu are at roughly 250 mm³). Definitely wouldn’t recommend them as a first printer, but Vorons are shining examples of what’s possible with open source (and they’re absolute workhorses too).
* Highly recommend them with glowing praise for being all around fantastic (they also run sales on the regular, though kit deals are usually reserved for special occasions like black Friday, or Clee day)
The new hotness in the open source MMU space is ArmoredTurtle’s BoxTurtle project, which seeks to be an open source equivalent of the Bambu AMS. I’m still working on the parts for mine, but from everything I’ve seen the build is way less fiddly (and as a result more reliable) than the ERCF.