Maybe the problem is that Wizz does not offer holiday packages but so does Ryanair ? Maybe the success of Bristol with its multiple frequencies to the various sun destinations ( I’m guessing Bristol may even have more than Birmingham on a daily basis ?) makes competing even harder. If Ryanair could have I’m pretty sure that they would have based ages ago at Cardiff .
Here is my suggestion. Stop obsessing what other airports are getting , stop thinking about based aircraft and encourage existing and loyal airlines such as Tui and Vueling to grow again on what Cardiff does best - Spain and encourage Ryanair to gradually grow its presence by flying ‘w’ patterns from other bases as they already do so maybe encourage adding Palma , Alicante , Tenerife and Lanzarote and who knows like Bristol it may in the future grow into a base.
all airports suffer setbacks - Bristol with BA connect and Bhx with Monarch but they got back up. Cardiff can too by working with what’s already there. There’s a market just have to grow it