Definitely avoid Bambu, they have been very hostile towards open-source developers! And this is speaking as somebody who owns an A1 bought before the ecosystem lockdown shenanigans (my printer is now in LAN-only mode to allow continued use of OrcaSlicer).
The Elegoo Centarui Carbon seems to be an excellent option from everything I've heard online. They recently released the "Canvas" multi-material unit, which looks pretty interesting, but the reliability of it is undetermined. It works fine now, but there hasn't been enough time to see if it will last. Could be a thing to consider if you will be using the printer for anything important! I've also heard that Qidi's Q1 Pro is excellent, and their new Q2 is the successor to that with support for their "Qidi Box". It looks like a neat option too.
Although much better than Bambu, as they support stuff like OrcaSlicer and OctoPrint out of the box, they aren't the most open. In both cases, they used a custom version of Klipper that is not open. I do know that there is the "OpenCentauri" project that is working towards an open Klipper for the CC, but full support (esp. for the Canvas) is not there yet.
Ideally, if within your budget, the Prusa Core One will be the most open option you can get. Great customer service, best in class build quality, repairable, etc. It supports the MMU and, in the future, the INDX for a toolchanger!
