Sounds like OD was out of water while motor was running? u may have damaged water pickup impeller? Was RPS correct range while OD down in water and shifting between gears?
If you have tried everything recommended and achieved 0 results, it is your only option. I have rebuilt several q-jets and Hollies over my life. If they get internal corrosion or blockages, sometimes it is better to just trash them.
It depends on what part of the country you live and do you continue using boat 12 months a year. Some on this forum will scream heresy but flushing with fresh water after using boat in salt water, many of us here just use a couple of light bulbs in the bilge area if we know freezing weather is...