Therea plenty of evidence that this isn't really all that true, and that if there is even a modicum of apparent reward for extra labor, people will continue to put extra labor in for that reward, often even past the point where that extra nominal reward seems to provide any additional real utility for them.
> . I'm already working less than 20 hours per week (pre-COVID) because my mortgage is paid and I don't really need the money anymore
Very few people, even well into the income range where research shows no additional subjective utility to additional earnings, ever reach the point where they recognize that they don't need any more money. So, you aren't typical and your entire argument seems to rest on assuming your atypical attitude holds generally.
> But as a society, if a lot of people like me slack off more, the real economy will suffer.
Anthropomorphizing the economy here obscures rather than aids understanding; what would happen if abstention from additional effort were excessively common is inflation, bringing down the real value of the UBI and reducing the number of people who found it sufficient to not exert additional effort, pushing people back into the workforce. It's a self-controlling negative feedback system.