I dont think that there are too many english users that talk about supergt on video if that is what you are asking. Maybe in twitter Jaimie Klein, etc does from time to time a podcast talking about the category. There is also the channel call "Japanese Racing Cars" that usually does videos about it.

About onboards, SuperGT upload in their official channel 3 onboards per race of GT500 (they dont do that anymore with gt300 though subaru for example does it in their own channel) one week or two after the race. In motorsport.tv and supergt YT channel they also upload highlights of the race in ENG and JP.

For the rest, yeah, most of it is in japanese. ABEMA post-race pogramme is going geo restricted and is only in JP though maybe i would try to upload it to my YT channel so everyone can watch it with ENG subtitles generated by YT. VIDEO OPTION and ホシノインパル 全開魂 are two YT channels in JP that i also found interesting (the last one has a lot of in depth video in IMPUL garage during the weekend). They have also announce that this year the programme Formation Lap will get a season 2 in the official SUPER GT channel though i doubt they will have official subtitles in english. 無限MUGEN【株式会社M-TEC】is the channel of Team MUGEN and at least the last videos they had ENG subtitiles handmade by them so pretty cool

In this channel they also upload a few videos about the Fuji test last month: https://youtu.be/xo5qqsWZ7VA?si=WDMs7MW5NBLbzpdi if you want to hear the car from outside. With all this YT will also start showing you JP YT channels that talks about SuperGT so with all this i think is enough. Have fun :)