Unsure what to think on this. Harry Potter as a franchise and the products itself aren't transphobic, and how much JKR and the artist and their art are to be connected is up for every individual. Declaring the Harry Potter franchise a "NoGo zone for anyone pro trans" is doing more harm than good. There were streamers viciously attacked just for streaming Hogwart's Legacy while trying to secure funds for trans organizations. Creating a fandom where outed pro-trans people will be squished by both sides.
The discussion is poisoned and after the Hogwart's Legacy fiasco, where way too many pro-diversity people received attacks, going down this path doesn't bode well. And lots of it is a low-hanging fruit people go for some virtue signaling. The boycott declared here doubtfully matters to those voicing it out. At the same time I go to local merchandise, geek and book stores which are filled with various Harry Potter gear, don't seem to get any meaningful attention, while making likely much more money than this. Furthering my belief, that the intentions are empty, which in turn wouldn't surprise me, as actual trans people are one of the tinies minorities out there and probably don't even have the majority inside these groups.
Attacking the products behind this, also attacks the collective workers and teams behind it, which create them hopefully with peaceful intentions and messages for a multitude of cultures.
Rowling receiving royalties on them isn't the mean evil. It is one small drop of a currently nepotistic, capitalistic, plutocratic society we live in. Single people should not have all that power to them with bigger wallets, influencing society like this. More money from Rowling, as with any other rich personas, should drain more aggressively to democratic institutions.