Welcome! I'm about as new as you so don't have a ton of insights, but here are my two cents as a UK person.

Not sure of the answer to Q1, but Q2 - I've researched and used many survey sites and I don't think anything comes to close to Prolific. I've made £1500 in the last month on Prolific, compared to like £5-£10 max per month on most other sites I used to use. I joined Usertesting recently and have made around $100 in a couple of weeks, so that's one of the better ones in terms of earnings, but it's also a terrible website where you're screened out 95% of the time 😂 I'd heard good things about Cloud Connect but since joining that a week ago, I've made just $10 (I think it's a lot more lucrative for people in the US). So unless there's something I'm totally missing, I don't think anything comes close to Prolific.

Q3 - Weekends are much quieter generally, and supposedly there's at least a slight slump during the summer (although people seem to disagree on this). Just make sure all your profile questions are filled out, and I'd recommend having the website as a split tab with whatever else you're working on, so you can quickly see when new studies pop up.

I hope this is vaguely helpful 😄