This worked for me when I was needing to verify it was present and had the correct message. I probably didn't need the selector in the .contains() but it works.

cy.get('ion-toast').should('exist').shadow().contains('.toast-message','Invalid username or password.');

The .shadow() method was the magic to see the internals of the toast.

The problem you face is that the toast appears slowly (compared to test running speed).

TLDR: Use cy.contains('my-selector', 'my-text')

Using cy.get('my-selector').contains('my-text') retries the text my-text, but not the existence of my-selector.

Using the format cy.contains('my-selector', 'my-text') will wait for the element to appear, then check it's text.

So, for a toast that has id="toaster"

it('Login with invalid password', () => {
    login.enterEmail(testData.username)
    login.enterPassword(testData.invalidPassword)
    login.submit()
    cy.contains('#toaster', testData.invalidCredentialsError)  
})

Or pass the error text into login.getToastError

it('Login with invalid password', () => {
    login.enterEmail(testData.username)
    login.enterPassword(testData.invalidPassword)
    login.submit()
    login.getToastError(testData.invalidCredentialsError)  
})

// login - approximate revision

getToastError(errorMessage) {
  if (errorMessage) {
    cy.contains('#toaster', testData.invalidCredentialsError)
  } else {
    return cy.get('#toaster').invoke('text')
  }

Assuming that login.getToastError() translates into something like cy.get('selector') then you can do something like this:

For partial text match:

it('Login with invalid password', () => {
  login.enterEmail(testData.username)
  login.enterPassword(testData.invalidPassword)
  login.submit()
  login
    .getToastError()
    .should('be.visible')
    .and('include.text', testData.invalidCredentialsError)
})

For exact text match:

it('Login with invalid password', () => {
  login.enterEmail(testData.username)
  login.enterPassword(testData.invalidPassword)
  login.submit()
  login
    .getToastError()
    .should('be.visible')
    .and('have.text', testData.invalidCredentialsError)
})