Agreed with both of you. 2nd/3rd choice keeper is an unusual spot to occupy. Zion Suzuki might make sense as he can be coached, develop behind Onana, maybe go on a 2-3 year loan in a couple of years (ala Courtois, Henderson etc.) and then come back to compete with Onana when the latter is 32.
I'd certainly be much more comfortable with Suzuki as number 2/3 if Heaton were there too.
I don't think we can spend much more than 5 or at max 10m on a backup keeper but it's looking likely we'll need 2. Maybe even 3 if Heaton goes. Johnstone isn't really feasible due to the aforementioned reasons. Maybe someone like Alex McCarthy, Jed Steer or Karl Darlow (shudders) come into consideration therefore, due to the home grown rule.