Insufficient storage popping up when transferring app to SD card
It's very possible you bought a fake SD card.
The markets are filled with unscrupulous people and businesses selling fake cards (bad eBay sellers are notorious for this).  They take 2GB-4GB cards, and make them look like much larger cards using illegal tricks.
See this link for even more details:
https://www.rmprepusb.com/tutorials/-fake-usb-flash-memory-drives
Update: In the comments, the OP indicated that they have good reason to believe their SD card is good (it previously held 25GB of data).  As such, I think it's likely their card is legit, but I'll leave this answer up, as it might help others in the future.