We need money for a large-scale advertising campaign
What is more important broader reach or better conversion? I bet on the latter. Reach is not endless. Sooner or later, you will hit the ceiling, and you will have to start working on the conversion anyway. So why not doing this before you spend tons of money on increasing your reach?
What is the most effective way to double revenue? If you think that the answer is “to double the reach of our advertising” think again. An increase in the conversion from visitor to a buyer, for example, from 0.1% to 0.2%, will also double your revenue.
The only downside of working on conversion is that you will have to do the work on improving your product, which is much harder than spending money on advertising.