Summary table by year with start value from different table

Hello all, I've been searching for a way to do this and can't come up with a way to do what I'm looking for. 

This post is close, but doesn't get me to where I need to be:

Solved: Creating a combined summary table - Microsoft Fabric Community 

I have a table of accounts, and a table of activity for those accounts and I want to generate a summary that I can summarize by Year/Quarter/Month. The problem is that the account's opening value isn't captured in the transaction, so I essentially need to take the oldest starting value for the account, then attribute that to the correct period.

The easiest would be to get the initial starting value into the "Net Change" column, but I can't modify the data source. 

Thoughts on how I can do this in Power BI?

Here's a sample of my model:

Accounts table

Transaction table

Account history

Here's the summary data I'd like to see and roll up by transaction date (year, quarter, month, etc)

Desired summary

[EDIT] Here's a link [Dropbox]  to the sample file I drew up if it's helpful