As of 1.17.2, there is no way to farm renewable copper with decent rates on bedrock. Converted drowned farms from a spawner, zombies filtered from a generic mob farm, and ocean drowned farms, all give abysmal rates for copper. In Java edition, players are able to use the zombie reinforcement mechanic, which allows them to get decent rates for converting drowned. With Bedrock missing this feature, there is a significant difference in how bedrock players are able to farm copper.