I'm not going to judge people for reading HP still, I know trans folk who do. We all draw our lines where we can.
As others have said, coming into threads like these to defend your right to do so is unnecessary. If you read a thread like this and see someone saying "anyone who reads HP still is not an ally", you don't need to respond to it. Trans people and people who care about trans people having a limit to their tolerance of this stuff is perfectly reasonable. Their frustration and anger isn't hurting you, you are not a vulnerable class of person because of your desire to read these books, you don't need to argue with them about it.
If you truly are an ally it shouldn't matter. Keep it to yourself and carry on being an ally.
Of course, if it comes up in your personal life and you feel you're being unfairly treated or judged, you have every right to stand up for yourself. There is no need for you to insert yourself into a discussion and get defensive, though.
I don't know dude... Do you put gas in your car? How can you do that while still being concerned about climate change? Do you eat animals? How can you do that while caring about any living thing?
The answer is people contain multitudes, the world is complicated, and we make the choices that make the most sense for us and our happiness and balance it with the rest like our morals.
I personally think it's enough to be an ally and advocate in your own way and you can do that while still liking and engaging with this IP. Putting a dollar into JKs pocket at this point is like adding a grain of sand to the beach.
Sorry, but this is not a good argument.
We all do what we can, yes, and many things come with a level of hypocrisy if we stand up for one thing over and other, but we also need to acknowledge context.
The current climate of transphobia is incredibly dangerous. Trans rights are being legitimised as a "debate" and "a matter of opinion". Rowling has a huge platform and is using her position as a woman who has experienced trauma to bludgeon trans people.
This is a crisis point situation where her talking points are literally leading to trans rights being rolled backwards by framing their right to exist as an opinion, and that is being weaponised by people on the right who seek to capacitively deny their rights by looping it back through those on the left who have been taken in by arguments put forward by people like Rowling.
Trans people are being actively harmed, and if we don't stand up to it now it's going to get so much worse.
The context here is that this is something that requires an all hands on deck approach from all allies, right now. I personally am not going to judge anyone for reading HP, but keep that to yourself. Coming into threads like these and arguing away the immediacy of the need to stand up is harmful.