Which plan for someone restarting after an extensive 6 months break?

I did my last ride mid May and then it was followed by a small surgery (which needed a 2 months stop to riding) and then just post-rest blues with very little activity.
Which plan would you recommend, that also includes some core exercises?

Try Fitness Kickstarter. In plans, go to Cycling>General Fitness>Fitness Kickstarter. I’d add Yoga. It comes automatically with some strength workouts. Both of those will hit your core (and other parts). If you want more core after a couple weeks, you can substitute for some of the strength or yoga workouts (or just add addtl workouts) some of the more core centric videos. You’ll find a bunch in Yoga and a bunch in Strength.