I have had good luck with a super low vacuum on testicles for extended periods of 20-40 minutes a few times a week, + a soft silicone ring for a few hours when not pumping.
I used to have very bad (Like where did they go?) and frequent retractions but over a few years of using a silicone ring to stretch testicle skin and the pump I have no retraction now, my testicles also seem a bit larger as well. though I think the testicle size may be more related to improvements in hormone profile/lifestyle improvements. All that being said, having more skin and no retraction gives the testies more room to breathe for sure and looks much larger just not being retracted.