Power supply to starter, or, cable condition. Probably what it was all along...........need to check main engine ground too.
I see nothing still of battery load tested. I replaced plenty of crap batteries that failed in less than a year, often with people who insisted on buying the too cheap (less CCA) battery. You play, you pay................Battery needs load test at correct CCA. Could easily cause that. In heat like now our battery replacements went up like 400%.