I was having this same issue on my ipad; receiving and sending android texts had been working, but then suddenly stopped. I researched everything, all the threads, etc. It came down to the option of Messages/Text Message Forwarding on my iphone—that option was missing all of a sudden. According to everything I had read, it should have been there, but it disappeared completely.
I tried following all the steps, turning my iphone and ipad off and on, making sure they were both signed into the same Apple ID, etc. The only thing that finally fixed it was to go to Messages/Send & Receive—on my iPad—and remove my phone number from the “you can only receiving iMessages to and reply from” list of devices. I got out of that screen and then right back into it and added my phone number back as an account I could make and receiving iMessages to and from.
After I did that, I started getting the messages that I could start using my phone number on my ipad, etc, and that added back the option—on my iphone—of text message forwarding (Messages/Text Message Forwarding). Once I did that and added my ipad back in, everything works fine now. Might try that on your Mac and see if that makes any difference. I think the Text Message Forwarding thing is what’s causing the problem, and for some people, that option is disappearing from their iPhones! So weird.