Kinda thinking out loud here, but if I average 1 month of vacancy per 13 months (12 month lease and then 1 month lost rent looking for a new tenant), that is 7.69% vacancy. Using round numbers, on rent of $1000/month, it averages out that I'm actually receiving $923.10 per month to use (rest should be going to used for vacancy reserves and is important for your business proposition). If I am using a property manager that charges 10% of rent, that costs $100 bringing what I'm receiving each month to $823.10. If the normal fee for placing a tenant is 1/2 month's rent, that is $500 every 13 months or $38.46/month. Now my monthly amount is down to $784.64. So as a rational landlord, $784.64 is what I should be using for considering your business proposition. In practice, how many landlords will agree to be guaranteed $784.64/month on a rental that rents on the open market for $1000/month? I'm guessing most would not take you up on this.
From your perspective, could you even make money at $784.64/month? That is the actual costs based on these assumptions. The only way to make money is to do better than the assumptions. Tenants stay longer than 12 months before moving out or renting it faster than 1 month without rent. Or you have to guarantee the landlord less than $784.64/month. Maybe $700/month so you make in the neighborhood of 10% each month? I think that would be even harder to get a landlord to agree to accept.
Switching back to the landlord's perspective, let's say you did find someone willing to agree to $700/month in this scenario. Looking at the 50% rule, we have 17.69% (10% management and 7.69% vacancy) of those expenses covered in the $300/month we are giving up to you for vacancy and management. That leaves 32.31% of the monthly rent in expenses left, or $323.10. $700 - $323.10 = $376.90. That is what is left over for principle and interest payments and cash flow for the landlord. On a property that rents for $1k/month, unless the house is mortgage free, it's hard to imagine the landlord making much money in this situation if they have to make P&I payments out of that.
So I would say your target market is landlords with free and clear properties or newbie landlords who think rent received - mortgage payment = pure profit.
Back to a landlord's perspective, why would I need you to guarantee my Class A rental where families stay through the school years and there is a waiting list of people willing to move in at 12:01pm when the previous tenant is out at 12:00? There is no reason to insure those rents. But I would LOVE for you to guarantee my monthly rents from the Class D shack that is in the hood I bought for $20k and rents for $1k. Where tenants occasionally pay more than just what it costs to move in. Adverse selection would seem to be a pitfall you would need to have to watch out for in guaranteeing rents.
I don't see this working out for both parties for any extended period of time.