You might want to double check this, but as far as I remember both the Sovol SV06 and SV08 are open source. The SV06 sounds in line with your desired budget, IF I remember correctly the open source thing. And as others have said, Cura, Prusa slicer and Orca are open source and cross platform.
SV07 then? It’s pretty full optional, but I believe they didn’t open source the whole thing.