Personally I always thought if Toriyama wrote it then it is canon. There is plenty of retconning and all that, but you can make it work if you try hard enough. Essentially what ever is most recent overwrites anything it conflicts with. Super Saiyan 4 is probably the hardest thing to fit in right now, but he definitely connected it to SSJ God with the red hair and eyes. It wouldn’t be that farfetched that he would forget it. Same exact thing happened with Ultra Instinct in Super. He only used SSJ4 twice, and only one of those times was his own doing. I also don’t recall how long the time gap is between Daima and Super

There really is no canon after Z. GT, Daima and Super are mutual exclusive.

GT and Daima can’t happen in the same timeline since Goku would unlock SSJ4 twice.

Super and Daima can’t happen in the same timeline, because Shin defused with Kibito thanks to Buu in Daima and thanks to the Dragon Balls in Super. Ignoring the different methods, he just can’t defuse twice. Also, as you said, the new forms don’t match and some other lore differences too.

GT and Super of course can’t happen together because of the different forms unlocked in each one.

All three also introduce exclusive lore about the Dragon Balls - their origin, function and creators.

As a cherry on top, there are three different Super canons: movie, manga and anime.

Everything is jumbled together in Heroes.