The dosage is a lot lower for malaria prophylaxis. But for RA and Lupus treatment it's actually about the same as doctors are using for covid-19, and they have been prescribing this for years:

Quote
COVID-19: 400 mg (=310 mg base) q12h for 2 doses, followed by 200 mg (=155 base) q12h for 5 days has been suggested by in vitro study (PBPK model) OR 200 mg q8h has been used in one small study without a loading dose[1].
Rheumatologic diseases
Lupus erythematosus: 400 mg once or twice daily for several weeks to months, depending on patient response. Maintenance therapy is usually 200-400 mg daily to minimize toxicity.
Rheumatoid arthritis: 400 to 600 mg daily. After 5-10 days increase the dose gradually to the optimum response level. When a good response is obtained, the dosage should be reduced to 200 to 400 mg daily.

from Jonhs Hopkins ABX Guide