I let them stack and don't worry about it. I remove maybe 10-20% of old growth that is tired, burnt from frost, or diseased and then I tip the good looking old branches. DF fruits on older arms.
If you remove young arms, it will make it grow more arms all over, imo... Each node can be a fruit, or it can be an arm. You want fruit!
I think your plants will flower on the branches that are hanging down this year. They look old enough. Consider tipping them.
Edit: I forgot to add that when they start to make little branches below the trellis, I do flick those off before they turn into anything. Anything above the trellis I let it fly.