As Marco said, you shouldn’t do anything illegal. In my not-a-lawyer view, the two issues there would be (i) doing work that isn’t covered by your work permit while physically in Taiwan and (ii) not declaring the income in Taiwan.
I’ve read quite a few threads on here previously regarding issue (i) – you’d have to use the search function and sift through them, as I can’t be bothered doing that again – but my understanding is that while this isn’t legal, it’s not uncommon for ARC holders to sometimes do occasional work for overseas clients (and sometimes clients in Taiwan, like small translation jobs for government bodies, including NIA) where getting a work permit in Taiwan wouldn’t be possible, and the government doesn’t usually actively pursue that. I don’t think receiving the money into a Taiwanese bank account would be the best option though (FYI, I’d also look into alternatives to PayPal with lower fees, such as (Transfer)Wise).
Issue (ii) can be easily solved by declaring the income on your next tax return and paying tax on it. My impression is that many people, including Taiwanese friends I’ve spoken to, don’t declare it, but legally you should – I’ve heard that the tax office staff are also a bit surprised when people voluntarily declare overseas income, but they’ll happily tax you on it.
I don’t know whether that’s likely to later cause a problem with NIA when applying for an APRC. My understanding based on stuff I’ve read is “probably not”, but it’s also not something I’ve really looked into as I don’t intend to apply for an APRC.