Hey there @DCrinnion1! Interesting use case here. I know HubSpot has had limitations over the years with managing ARR. Let me see if I can help here a bit.
I think that to do this without Operations Hub, you’re on the right track with custom properties. As Pam mentioned, workflows will also help. Plus I’d look at calculated properties.
Here’s what I’d start with based on some research. I’d start with these custom properties.
- Create a custom property on the Subscription object called “Subscription Order” (Number).
- Create a custom property on the Deal object called “First Subscription ARR” (Number).
- Create a custom property on the Deal object called “Subsequent Subscriptions ARR” (Number).
Next I’d suggest three workflows.
First: Set Subscription Order.
Trigger: Subscription is created
Action: Set “Subscription Order” to the count of associated subscriptions for the deal
Next: Update First Subscription ARR.
Trigger: Subscription is created or updated
Conditions:
Subscription Order = 1
Subscription Status = Active OR Scheduled
Action: Copy Subscription ARR to Deal “First Subscription ARR”
Then: Update Subsequent Subscriptions ARR.
Trigger: Subscription is created or updated
Conditions:
Subscription Order > 1
Subscription Status = Scheduled
Action: Sum of all Subscription ARR where Order > 1 and Status = Scheduled, set to Deal “Subsequent Subscriptions ARR”
With all of this, you should be able to have some calculated properties to help with reporting.
- ARR (Calculated): Formula: [First Subscription ARR]
- Forecast ARR (Calculated): Formula: [First Subscription ARR] + [Subsequent Subscriptions ARR]
This setup should ensure that the first subscription’s ARR is always included in the ARR field, regardless of its status, and subsequent subscriptions are only included in the Forecast ARR when they are scheduled.
One final thought. To help handle deals closed with future subscriptions, you can create an additional “Close Deal Workflow.”
Trigger: Deal stage changes to Closed Won
Action: Set the first subscription’s status to Active
This will ensure that when a deal is closed, the first subscription is always considered active, even if it’s scheduled for the future. And of course my usual advice to clients when I suggest a solution: Remember to test these workflows thoroughly and adjust as needed for your specific use case 
I hope this helps!