In my job, I have Projects, each of which have multiple Purchase Orders and Invoices and a Approved Budget.  
One of my tasks is to provide a MONTHLY update at the PROJECT level as follows:

1) Purchase Order Forecast - how the Project Budget will be spent, i.e. in WHAT MONTH will WHAT VALUE of Purchase Orders be issued to vendors. Total of all months must equal Project Budget.
2) Invoice Forecast - how the Project Budget will be goods-receipted, i.e. in WHAT MONTH will WHAT VALUE of vendor Invoices be receipted/approved for payment. Total of all months must equal Project Budget as well.

As the Projects are long-term, I have Purchase Orders that can receive monthly progress Invoices for 18 months or longer, so it gets complicated to track at the Project level and I have no idea how to tackle this.

I typically use Excel, with Project IDs down the left and months across the top.  I have to keep two sheets: one to view each forecast (Purchase Order and Invoice.)  
Once a month has passed, that column becomes the ACTUAL data for that month and I am asked to explain variances on a Project Level, but I need to refer to the Purchase Order level variances for true context.

My thought was to build the forecasts into my database at the Purchase Order level, and then aggregate the monthly data to the Project Level from there.

Assuming I allow for 24 months of forecasts, my Purchase Order table would quickly jump from 9 fields to 57... that can't be the best solution, right?
Example
OrderT fields
OrderKey
VendorKey
CurrencyKey
PONum
IsActive
Notes
CreatedOn
POForecast Sep22
InvForecast Sep22
POForecast Oct22
InvForecast Oct22
and so on through the following 22 months

I've searched and searched but I struggle to find helpful results because "Forecasting" typically means sales projections and is revenue-related, but I need to plug in real values and there's no trend line to be found in my data.

BONUS QUESTION
I also work in seven different currencies but need to subtotal in USD by Budget Category

Thank you SO much for even the smallest amount of help.