How to teach 16 week old to not get fixated on and bully 12 week old

My 16 week old kitten Henk is the uncle of my 12 week old kitten Koos (fun fact). I initially wanted to adopt them together when the youngest was old enough, but the oldest was the last one left of his litter and played too rough with the younger litter. So he had been home for a couple of weeks before I took his nephew home.

It's been a month since I've had them both in my home. I believe they get along fine, there's no hissing or growling on just seeing each other. They can share a liquid tube without trouble, eating wet food on either side of the door also goes without any troubles. But from the start the oldest was playing too rough with the youngest so I've had them in their separate spaces, Henk in the livingroom as that is the space he was used to, Koos in the hallway to get settled down.

Every day I have a couple of play sessions in the hallway, in the beginning the oldest would get rough pretty quickly. If he got rough I corrected but after a while he would just get frustrated/energetic and get more fixated which is when I move him back to the livingroom to cool down. We can currently get about 30 minutes of fair and nice play (depending on the time of day) after which I have to separate them again.

The youngest has been wanting to explore the livingroom. I've organized one succesful swap where Koos could explore to his hearts content for two hours and Henk sniffed up all the smells in the hallway and ended up napping on Koos preferred napping spot. However the next couple of times that I've tried the oldest gets very frustrated in the hallway staying around the door and meowing continuously which than distracts the youngest from actually exploring and leaving his scent.

Trying to just let Koos in the livingroom while Henk is there also hasn't been succesful as Henk already runs towards the door when I go there. The few times he just stays laying down he immediately runs for Koos at the first sign he's near and keeps being fixated on him and chasing him the only thing that seems to really distract him is a liquid tube. Toys with bells only get his attention a little bit but that also attracts the youngest which results in Henk immediately going for Koos, I've tried two separate wand toys but it's not very practical. I have had a pheromone diffuser in the livingroom for about two weeks now and I've also had a washcloth in the sleeping spot of Koos, rubbed it on his body and mouth and rubbed it all over my furniture.

I've called the vet today to discuss whether 'early' castration could help as I believe Henk is displaying territorial behaviour. My vet usually castrates around 6 months, but wants to do it earlier in cases of behavioural issues (if balls can be seen). She also explained that it just takes time and to let them be together and figure it out on their own and boob/slap his nose when he bites the youngest in the neck and move him away (separate room) if he doesn't listen as that is what mom cats usually do. But I don't see how this works with the continuous chasing and pinning down when he gets the opportunity, also I am scared he'll just become afraid of my hands. Moving him to the hallway also doesn't seem to work (or maybe I'm too impatient and keeping him separate for too long) but the minute the door opens he full on races into the living room to look for his nephew.

Anyway tomorrow morning we'll be saying goodbye to his balls in the hope that it will get rid of some of the behaviour. Of course it will still take a bit before the hormones are fully gone so any advice or useful tips and tricks for in the meantime (and possibly after if castration isn't the fix) are very welcome.