looking to become a serious keyholder

I think Chastity is an important part of BDSM. I guess you tie up your boyfriend when you are playing but at the end of playtime you have to let him go. With chastity he is restrained and under your control for as long as you want. So get a cage and insist he wear it. Gradually increase the time he spends in it. He will become desperate for release and will obey your rules in the hope of release.
If he begs for release that is a great reason to punish him with a good caning. Also add time to his lock up.
 
Happy to offer advice on any specific questions.

Start with a basic cage that fits well. That is, plastic/printed, with the cage length matching the flaccid penis, and a variety of ring sizes. Start with lockups of a few hours and check for marks.
 
Chastity has truly deepened the bond between my husband and me. It’s brought a level of trust and closeness that we hadn’t expected but now can’t imagine living without. If I could offer one tip, it's to let him be part of choosing the cage. It makes all the difference. Just recently, he showed me a stainless steel chainmail cock cage from Lock the Cock. He said it looked wickedly medieval. I surprised him and bought it, and the smile on his face said it all.