I have a couple domains I would like to move over to proton mail and would pay the yearly fee but need the app to support multiple addresses before I do. I have a business email, personal, and others.

While ProtonMail is implementing support for multiple account you can use Parallel Space. Works perfectly and receive the nofications.

I've been hoping for a while now to see this idea get implemented into the Android app. It would be super slick to be able to add multiple accounts into the app just like the Gmail or Outlook apps do. I too use multiple accounts, each one serves its own purpose and would like to be able to get notifications from each of them on my phone whenever I get a new message.

I agree. I have multiple accounts each for a different purpose. It would be beneficial to be able to receive notifications for them all and be able to quickly switch between them in the app. For instance I have my main account which I'm trying to keep as "clean"as possible so I only give it to people I know. While I have another account I use for signing up for services which I know will result in lots of useless emails. I'd like to get notifications on my phone when I receive mail in either account but I want the app to display only one inbox at a time. Thus the ability to quickly and easily switch between accounts would be very beneficial.

Not everybody needs all the features of ProtonMail Plus. Like my wife does not want to use a smartphone so it would be nice to have the option of logging into multiple accounts on the same android app!

I would argue if you want multiple accounts, i.e. multiple email addresses, that it would be good to upgrade to a ProtonMail plus account.

I have two ProtonMail accounts but can't add second ProtonMail account to Android app. :(