Textjoin function based on changing row number

I am trying to create a text join from this raw data source.

I want to create a 'description' field in another sheet that 'textjoin' the description from Column G.

So it needs to textjoin G2 - G8 for the first title, then based on where the second title is in column B, Text join G9 - G15.

This is hard because there is a different number of rows for each description based off the title in column b.

Is there a way to textjoin rows with key BTRTN-1573 or blank, until BTRTN-1569, then start new text join in the row below?

So . the description would be for the first one: "As a member of the team, I want to do something, So that I can do something else, Success criteria:, Completion of the formula, No dependence on others"
And the the second one: "As a member of another team, I want to do something else, So that I can do something again, Success criteria:, Completion of a different formula, No independence on others"

Currently all I can figure out is how to do one at a time with this formula:
=TEXTJOIN(",",TRUE,G2:G8)

Thinking something like:
=IF(B2="",=TEXTJOIN(",",TRUE,G2:G8),"")