Golf gifts are tough because everything is incredibly personal. Unless you are 100% confident and know something specific, I'd stay away from anything he uses on a regular basis. He either already has it or has a strong preference. This even applies to the #1 ball in golf.
Gifts for successful people are hard because the goal should be to find something they wouldn't buy for themself. The problem is they've already purchased most of it.
Balls - if he changes clubs every two years, he's likely picky about his golf ball. What if he just switched to the AVX at his last fitting due to a drop in swing speed? Best case, he does play the Pro V1, but it's already something he has and buys for himself. Balls can be expensive, but are still disposable. This may be an unpopular opinion but balls may not be as safe as you think (coming from the benefactor of a father-in-law who doesn't play the pro v1).
Bag - no. Most avid golfers have the bag they want or know which one they want next. Or have the bag they want, but already have their eye on another at the same time.
Range finder - already has it or knows what he wants if he needs a replacement.
Bluetooth Speaker - has it or hates music on the course.
Clubs - absolutely not. Even the gift card, he's already proven he doesn't hesitate to spend that money on himself. If he buys from his club, don't buy him a gift card to his own club!
Gift cards to local courses - is he a member of a club? If so, he probably rarely ventures elsewhere because of pace of play and his regular games.
Custom scorecard holder - this could be gold. Something not often used, but when he does it brings back good memories of the people who gave it to him. Would likely keep a long time.
Travel bag - club glove or sun mountain club glider. All depends on if he plans to travel AND the condition of his current one. As with the scorecard holder, he'd likely keep it a long time. You've worked with him for 18 years, is it possible to get intel from his wife?
Other option is to avoid golf all together. What's his drink of choice? Don't get the drink, get the whisky or wine decanter and glasses he'll use for a long time.
