Phone hacked... Mmmm, I think that every single one of the 18 cell phones that I have bought since October 2016, my ex-wife just hacked away at my phones like it was a game. I lost tons of files, data, photos, videos,.. well you get the picture. My life was miserable. I went from having the coolest, baddest phone to a flip phone.
I decided to fight back in her arena and I found out how she did it. I found numerous apps that have pushed me to be ready for her attacks. And I paid for a few online Google courses.
Of course, in your situation may be a bit different than all of mine, but I can tell you that Facebook, Gmail, Google Developer console, Google cloud console, are the tools my ex-wife had been using. You have to begin minimizing as much information, files, photos, contacts, as fast as you can and save all that to a different cloud than what you get from your browser. Next get a good system inspection app. One that does it all while you are NOT rooted. ( If you were rooted, a lot easier), an app like SD Maid by GitHub. Get the pro version.
As for your WiFi, I used FING. Excellent app that shows you everything about your network. Oh, that reminds me, check your settings on your router too. That someone may be right in front of you.
Antivirus/malware= I use es/et mobile security. This app found a tracker on my phone immediately after installation. Neither Norton or Malwarebytes even scanned it.
That's a beginning for you. I can go into this more in depth, but I think you have a good starting point with the information everyone here has given you. I went through so many phones because I never believed getting hacked could ever happen to me. I was wrong.
Good luck with your search..