I am surprised you were able to get the firmware compiled with core 3.0.0. It has been a while but I remember when I tried to compile the firmware with 3.0.2 I had to make a number of significant changes to even get it to compile. The main advantage of core 3.0.x is that it unifies the handling of WiFi with wired Ethernet (using lwip2). Since both use the same WiFi API functions, OTA update works with wired Ethernet as well (currently with firmware 2.1.9 OTA updates only works with WiFi as wired Ethernet uses a separate library). I will probably check in the changes I made to a separate branch on github. I made the changes as an initial step to prepare for firmware 2.2.0, but it’s quite far from being completed.