Having any visa does not mean exemption from questioning. There is a spotlight on SPERs as that is the most frequent option for scam applications so be prepared for questions even though you are legit.
The 500k does not need to be seasoned for 3 months. It depends on the embassy you apply from. If applying from London a final balance of 500k with several months statements is ok. You can see posts of people transferring money same day or day before and approved. Instant access to funds required
It is a good point about Taipei. When i applied in person for my Workcation DTV there, a few SP applicants were turned away for not having proof of online Income.
If you have your own students, you can create a contract. Online platforms usually have a teaching agreement which you can submit. No minimum income is required.