Around 90% of the people died of plagues.

It's really impossible to speculate how things would have progressed without those plagues.

Or worse, if Native Americans were full of plagues that the conquistadors would bring back to Europe to cull 90% of Eurasia.

It’s impossible to tell. If it had taken decades or a century longer because their numbers were higher, they might have had time to start up their own production of technology.