I’m not sure which threads you’ve read, but these ones and the ones linked therein may be useful starting points (there are others too):
The range of prices you mentioned in the original version of your post are similar to those I had in my head (I’m not sure from where though - probably discussions on here and/or Facebook groups, and they may be way off).
That is, from several thousand for a local accountant who isn’t all that familiar with freelancers working for overseas clients (I suspect that’s the norm here, and it wasn’t all that long ago that even tax offices here were giving incorrect guidance on what to do about this kind of income, i.e., counting it as AMT) to say NT$20k for one of the companies targeting foreigners and charging them a premium for it. Again, take these numbers with a grain of salt, but they’re what I have in my head.
If I’ve correctly understood your situation, I wouldn’t expect either of these to be able to significantly reduce your tax liability on freelance income from overseas. When I said it’s easy, what I meant is that you can basically sum up your income for the year and substract the exemptions and deductions. There aren’t that many of these - basically the standard ones that everyone gets, which vary with your number of dependents and stuff - and details can be found online or in threads here. Then apply the appropriate tax rate depending on what bracket your taxable income is in, and that’s about it.
What I’d suggest is totaling all this up closer to May and asking questions at the time if anything is unclear. There are some possible ambiguities like whether you should take the standard or itemized deduction and things that might vary with your line of work, but it’s really not all that complicated. It can be a bit intimidating the first time, but personally I don’t find the tax software all that bad, and if as you say you want to have as much control over the outcome as possible it’s in your own interest to understand as much as you can (again, for some of these things even the tax office and specialists etc. don’t always give 100% accurate advice anyway).
The tax office here also tends to be a bit more flexible than those in some other countries, e.g., with respect to what documentation is needed for overseas freelance income. It’d be annoying and expensive to spend a load of time and money getting everything notarized (as written in the rules) then finding out you could have got away without bothering (as in my case).