The JSON file is finished and can be used to update the Arduino boards manager. Add the following link to the Additional Boards Manager URLs in Preferences of the Arduino IDE:
Next go to the Boards Manager under Tools and search for ATtiny. You should get a result with ATtinyCore boards with
micronucleus by California STEAM. Click on install to install these boards.
It will download the ATtiny Core 1.2.2 from Spence Konde and Micronucleus 2.0a4 from Digistump. After a successful install you will find the new boards under Tools -> Boards.
You can use LTO during compiling to reduce your sketch with about 10% (depending on the sketch). Activate LTO by going to Tools -> LTO -> Enabled.