So, first, drop the squirt bottle. It'll break up the fights in the moment, but negative reinforcement won't change the cats behavior long term; it will just make him see you as a threat.
Second, how many litter boxes do you have? Litter boxes are an important part of staking out their territory, and having to share one litter box can make them think they're encroaching on each other's territory. Experts recommend one more litter box than cats in the house, but one litter box for each, in different parts of house, should be the minimum.
Next, how much are you playing with the younger cat? If he's stalking and instigating most the fights, he might just have play aggression that he's trying to get out on the older cat, who might not have the patience for it. Try playing with the younger guy until he gets tired, at least once a day.
Finally, look around your house and try to eliminate the areas where the older guy might get trapped. Give him escape routes, make sure he has a shelf or a piece of furniture he can not only jump to, but make an escape from. If there isn't enough vertical space, the younger guy is just going to run him down until he gets him cornered.
Sounds like you're killing it on the litter boxes. So, does this open floor plan allow for a cat to traverse the space without touching the ground? Having vertical pathways where your cats can make an exit without getting blocked on the ground is important for side-stepping conflict. I have a very patient cat and a surprisingly gentle toddler, and they get along very well, but he's often too excitable and loud for her. When he's in the dining room, blocking her path and getting rambunctious, she can avoid him by jumping on the dining room table, then to an island bar counter, then down to the kitchen counter and from there exit into one of the bedrooms. Similarly, in the living room, she makes use of the back of our L shaped couch to reach the two exits if she doesn't feel like engaging with the toddler playing on the floor. Some cat shelving, or even just some strategically placed furniture, can give them an exit route (doorways are tough, but maybe a small end table they can jump up on or something? Difficult to say without walking through your house).
It sounds like you've got a lot of enrichment for the younger guy. The only thing I could possibly suggest would be adding more structure. You could try playing until he gets exhausted in the morning, then do his morning feeding, which would probably get him to burn some energy and do a big morning nap. Then do the same thing at night to get him to calm down for bed. For all I know you're already doing this, but it's all I can think to suggest. It definitely sounds like the younger guy is instigating out of boredom and the older guy is just is not having it.