1) How many days before the actual expiry date should I go to the immigration?
You can go there 45 days prior to the end of the existing visa. However, I would suggest you at least 2 weeks before the actual visa expiration date you should go for an extension.
2)Is it still compulsory to go there in person despite the increasing COVID-19 cases?
Yes, visa extension can be only done in person. Please prepare visa costs 1,900 Baht.
3) If yes, do you have the address for me?
Immigration Bureau (Sub-Div. 2, Imm. Div.1) (Building B)
Address: Chalermprakiat Government Complex, 120, Moo 3, Chaengwattana Road (Soi 7), Thungsonghong, Laksi Bangkok 10210
The easiest way to get there is: First take MRT from Bangwa to Chatuchak Station and from Chatuchak get a taxi and go there. You can tell the taxi driver Chaengwattana Immigration Building B.
4) it is my first (and last) extension - when I go to the immigration, is this simultaneously my 90 days report, or do I have to do that additionally online or something
Your 90 days counting will start the day you actually extend your visa. So in this case you can ask the Immigration officer when is the 90 days report due date. They will guide you in that case.
Online reporting has to be the SECOND 90-day reporting or later on your current visa or visa extension of stay. This is important. You have to report in person or by mail on your current visa or visa extension once in the past before you can do an online report.
However, I suggest you ask the immigration officer regarding the 90 days procedure.