Welcome to the group
Make sure to drop in on the The Ironman Training 2020 Thread - #2886 by Alen as well
You ask some of the same questions we all ask, and I don’t think there is an easy answer
Is it worth doing the same plan over and over? Yes - as long as you see the gains, but borrrrrinnnngggg. Sometime changing up stuff prompts new adaptation. Also variety = spice of life 
I don’t lean towards the Century plan unless long rides are a limiter - take a look at the 40K TT plan for sustained power over time.
I don’t think there is a magic plan - I freaking wish - you will have to do the work, but the the main focus should be to raise fitness (FTP) - as the saying goes - a high tide raises all boats.
Mike