It's something that's plagued Merlin for a while, and they've got in some big doodoo recently for not having proper staff remuneration. My experience at a merlin park was much the same; manager worthy workers preferred to stay on 0 hours because they actually got paid the time that they worked despite unguaranteed work hours, whereas the permanent management team would have dozens of days in lieu owed to them by the end of the year, often in an irredeemable capacity.
A very noticeable element of enjoyable theme parks from a guest perspective is, when a park treats their employees well; they are far more efficient, professional, and more likely to enjoy the job, in turn enhancing the visitor experience.
The limited park openings could well be a staffing issue - with no second wave of staff coming in for break swaps, many workers will not be comfortable, or able to work more than an 8 hour shift daily. Once you take into account opening and closing checks and responsibilities, there is your 9-5 day.
A term I often hear thrown around these days is visits to merlin parks feel 'transactional'. You give them X amount of my time and money, and they give you their product. More and more frequently though, I hear of people having a less enjoyable time - The recipients of a bad product, with no refund. These days to me, visits to merlin parks feel like a scam. Last year I was lucky enough to visit Alton Towers for free, but it was unquestionably one of the worst visits I've ever had to a theme park, and it felt like a colossal, miserable waste of time. All the reputation places like Alton Towers and Legoland have as being these magical, vibrant places culminates in a depressing experience. This feeling is of course enhanced by the knowledge that many of us have as people interested in parks, of how things are ran elsewhere, how things are designed to be ran, how things used to be ran, and the nostalgic lens of fantasy cripples your experience in the moment.
Proper staffing, staff training, equipment, and staff treatment would be a seriously good way to jump start the heart and soul of the merlin parks when they feel so money-syphoning. It's expensive, and the UK struggles to have good incentives for lower income jobs such as being a ride host or operator compared to many other countries around the world... But when guests are happier, they're more likely to spend more money, and people actually being able to do their jobs properly, comfortably, and well, could realistically achieve that fairly quickly.