You shouldn't make your users choose between MAU supported and features.
Here's how I would do the price tiers, since you asked:
* STARTER - Free up to 5,000 MAU, features as listed.
* STARTER PLUS - Add a credit card and just pay $29 plus $0.05 for each additional MAU over the first 5,000. Same features as listed for STARTER.
In this approach, MAU 0 - 5,000 is free, and MAU 5,001 is $29.05. In the Starter plus tier, 6000 MAU is $29 + $50 = $79 and 10,000 MAU is $29 + $250 = $279
* For the PRO tier, the jump is pretty dramatic between $29 and $249 for the same # of users, so I suggest a 2x rather than a 8x premium. This means that you should price pro at $49 plus $0.02 for all users. This makes 5001 users $49 + $100 = $149
In this arrangement you don't even need to have a 5000 starter cliff for Pro. You can start with pro at even 500 users if you wanted the MFA features and the cost would be just $49 + $10 = $59.
Here's how I would do the price tiers, since you asked:
* STARTER - Free up to 5,000 MAU, features as listed.
* STARTER PLUS - Add a credit card and just pay $29 plus $0.05 for each additional MAU over the first 5,000. Same features as listed for STARTER.
In this approach, MAU 0 - 5,000 is free, and MAU 5,001 is $29.05. In the Starter plus tier, 6000 MAU is $29 + $50 = $79 and 10,000 MAU is $29 + $250 = $279
* For the PRO tier, the jump is pretty dramatic between $29 and $249 for the same # of users, so I suggest a 2x rather than a 8x premium. This means that you should price pro at $49 plus $0.02 for all users. This makes 5001 users $49 + $100 = $149
In this arrangement you don't even need to have a 5000 starter cliff for Pro. You can start with pro at even 500 users if you wanted the MFA features and the cost would be just $49 + $10 = $59.
Just my 2 cents.