i second the prolific recommendation
the volume of studies you get is variable, depending on demographic information and time of day/week/year
you get the most during uni term-time on weekdays in my experience
the money is also very variable, but they have guaranteed minimums per hour
i usually have a bunch of <10 minute studies that pay ~£1 (works out to slightly under national minimum wage), but about once or twice a week i'll get one that pays a fiver for like 15 minutes (over minimum wage)
sometimes i'll get one that's ~1 hour and pays £30 (triple minimum wage), but that's pretty rare
over the last ~3 years i have made £961.67, and i don't check as often as i should tbh, i could have made way more than that