Seems WA855RE didn't make it to 19.07, which would indicate it's a 4mb flash device.
If that's the case, you're not going to be able to fit any additional modules onto the flash
unless you create your own image, and delete some unneeded stuff in the process.
Might gain some additional space by replacing ath10k with ath79 though.
Or you can confirm the flash size have increased in v2.