Use a custom Invoice Groups field, don't store value, drive the result using a summary saved search on Invoice Groups joining to the Transaction table to create a map.

Then in your Invoice Group PDF template you can break apart the custom field result using

split

and then rebuild your Invoice summary hash using the mapping to add in any line specific data you need. Then just output your new hash into the items table.

Agree it's a pain but it's the only way I've found to accomplish it w/out needing a script

It is the same pattern I've used to get custom record sublist data into a standard record, like a transaction (e.g. rollup summary quantities for Bill of Lading PDFs)

I'm debating a custom solution for this - and trying to figure out how it balances against trying to explain what you are doing to someone else to have them try and butcher the PDF

More or less than 20 hours?

Assuming they are familiar with building their own hashes within freemarker, which can be somewhat maddening

Still, might take 10 hours if you run into a lot of problems and spin your wheels on it a ton