Whenever I book flights with LH Group, I have a problem that their booking engine suggests shorter layovers than I'd like. They are doable, but not comfortable and since I'm usually not in a hurry, I would rather spend more time at the airport than having to run for my next flight (particularly when it comes to transfer at FRA). I've tried using multicity option at LH website, but that shoots the price over the roof.

What is the best strategy to book flights with longer transfer times? Is there some way to make LH booking engine offer them? Or any recommended OTA that can sell it and offers good support when things go wrong?