Definitely Upwork because you can be proactive and pitch.
Everyone posting an ad are essentially warm leads who want to hire you, so just message 20+ people a day with your offer and you'll have a list of clients in no time.
Fiverr is the other good one, but you have to wait for people to come to you.
Meaning it's a slow burn and you get nowhere near the amount of work you'll need, especially starting out.
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