From my experience, these kinds of hybrid policies with mandatory "X days in office" are the worst of both worlds.
A mandatory "X days in office" means that during the office days everyone is running and on edge trying to do all their "office" things so you don't get all (or even most?) of the benefits of an entirely on-site company:
"Culture" and watercooler chat doesn't really happen if everyone's schedule is packed and they are on edge from having to commute, find a desk, etc. Those things only happen naturally if everyone is in a comfortable routine - it's not something you can force to happen and especially not by forcing people to go through a stressful and aggravating experience beforehand.
Equipment and desk space is a problem. You have to lug around equipment, which might be a problem if you rely on anything beyond a small laptop, and plug it in/set it up every time you arrive at the office and then do the same when back home. Hot desking is a bit of a pain as well if people don't even have permanently assigned desks. This ultimately means people may be constraining themselves and not operating at peak comfort/efficiency just to avoid having to also carry equipment (that would otherwise make them perform better).
Transport is also a problem - because you don't commute daily you end up paying full price for transport - most "season tickets" won't help as those only work out cheaper if you actually commute daily. It's also more stressful and disruptive to commute only some of the time as opposed to daily where you can fall into a routine and tune out some of the annoyances.
Even when it comes to the uncomfortable truth that some employees might be slacking off when remote, hybrid (as opposed to full on-site) doesn't address that either, since they can not only still slack off during the remote days, but the above annoyances would also eat into their productive output in the office days when they aren't slacking off. Furthermore, the above annoyances reduce the output of the non-slackers too, which outweighs whatever small productivity benefit they might gain from forcing the slackers to work during office days.
Pick full remote or full on-site (with reasonable exceptions - most jobs wouldn't care if you occasionally need to work remotely) and stick to it. Hybrid combines the worst of both worlds and is ultimately just there to introduce some "pain" back in the typical employee's life because it's uncomfortable to the higher ups that grunts got a significant boost in both quality of life and finances (as they no longer have to live near the office) and enjoy a perk that used to be exclusive to the elites.