The error was my first answer to this topic.
I jumped on the first obvious cause of your issue, without making sure I get all relevant information. I should have pointed you to this topic first:
Please read How to get the best out of this forum.
I don't have the actual Waveshare 4.2" b/w display. Therefore I don't know if it is supported by GxEPD2.
also compare the inking on the flexible connector, if visible. And report it.
I still don't know the inking your panel has, or if it is hidden.
I might buy one of their current version
I will do that, because this is one of most often used panel size, and the one that caused the most issues reported.
But even if I buy one, I can only be sure for a short time. And users still may buy variants from other vendors. Boards with the Waveshare brand logo are known to possibly be copies or fakes, or surplus not intended to get sold. And Waveshare doesn't publish the origin of the panels used, nor the controller and its OTP status.
But the BUSY times you report point to a different or addidional issue: the SPI communication most likely doesn't work.
A clear pin-to-pin wiring list would have been helpful.
Did you check continuity of your wiring, with an ohm-meter? I had one cable from Waveshare with a bad DuPont connector.
And I should have asked which board have you selected to compile for.
To compare if the board uses the default SPI pins you wired to.
Without knowing what you want to use an e-paper display for, I can't give a recommendation.
If you buy panels from an official Good Display shop, you know the panel name, and you can check if it is supported by GxEPD2.
-jz-