Welcome to Forumosa, and hopefully Taiwan 
Some quick answers and links to relevant portions of the FAQ.
30 days, but due to COVID sometimes 80. The big time sink is in verification of your skills.
1-3 years, you choose. Validity starts on card issuance and ‘renewal’ is simply going through the application process again - you need to again reach qualification level of any of the then available qualifications.
There are no minimum stay restrictions, but there are minimum wage restrictions if you get a job.
Helpful info in the FAQ, but basically you first get your card and then subsequently apply for visas for them.
You can work for whichever and how ever many companies you choose. The underlying purpose of the Gold Card is to allow you to work, if you don’t plan on working at all I’d suggest an alternative visa as there is precedent for cancellation of visas where it’s discovered the stated purpose of residence in Taiwan is false (eg students who don’t go to class).
‘renewal’ is simply going through the application process again - you need to again reach qualification level of any of the then available qualifications. For example, if you apply on the basis of your salary and then don’t achieve that salary during the duration of your card you won’t be eligible to use that same regulation to apply again.
Yes, you must hand in your physical passport at the local Taiwanese embassy. (See After passing skill assessment)
I would wait until 3-4 months before you plan to travel. You don’t need to fly in straight away after getting your card.