Dynamically merge two tables repeatedly

Bump (I got flagged for spam but thankfully got taken out of post jail).

Reading through other posts on the forum, thought I would provide some additional context:

New(ish) PowerQuery/PowerBI user. Familiar with data warehousing concepts but new to doing it in PowerBI/Fabric/Synapse/whatever MS is calling it this 5-minutes.

I'm very aware that the above is some combination of stupid question/repeatedly asked question in the forum, but I have spent some time searching and looking at related posts, and I'm seeing suggestions that are close, but not precisely, what I'm looking for (the answers seem to go off on a tangent compared to what I think I need.

I also don't know what will be the best option for performance:

Merge, Unpivot/pivot (tried this and boy was that going to look ugly), List.Accumulate (still wasn't clear on how that was going to work), or even doing it as DAX in a measure.

Basically, my very atrophied data warehousing muscles told me it's best to denormalize all of this into the main fact tables at data load, but I'm struggling to figure out how to do it for our use-case.

Thanks again, hopefully my overly simplified tables above speak for themselves in a simplified way.