Well, the most open you can get is Prusa's machines. Repairable, upgradeable, with great customer service to boot.
Other companies are more open than Bambu but few support the open-source movement like Prusa. Qidi, Elegoo, etc. all have great printers that I can recommend (Q2 and Centauri Carbon are fantastic options based on feature set) but they don't use a very open firmware. They are compatible with OrcaSlicer and aren't as bad as Bambu though.