I know the answer is probably “You can’t!”, but you never know:
I’m planning to install GrapheneOS or LineageOS and do a full deGoogle. The only thing I’m not looking forward to is saying goodbye to this fantastic app: https://play.google.com/store/apps/details?id=com.natureguides.birdguide
Would there be any way for me to reinstall it without using Google Play?
I think you can if you login to your google account with the Aurora store. You won’t be able to buy a new apps but I think you can redownload and update paid apps that method. I believe I read that or heard that somewhere we’ll see if I can find it
That’s correct, I have done it before.