I don't have any particular insights on the general problem, but I can definitely offer some help on prioritising my PRs. I've categorised them as follows: each category is in roughly increasing complexity both top to bottom and left to right.
Small and simple PRs: #1555, #1561
Simple to understand, but with a few dozen lines of code changed: #1551, #1543, #1537, #1542, #1544, #1560(!)
Simple to understand, but with a some feedback needed before merging: #1545(!)
Major changes to how hledger works, requires major discussion and testing: #1554
Simple changes, but depends on dropping support for GHC<8.6: #1557
I've flagged #1560 because it significantly simplifies the valuation/costing code, which would be helpful for both #1554 and @chvp's #1432.
I've flagged #1545 because it causes a significant change in the naming of ReportSpec's fields which is likely to conflict with other PRs, and also because the introduction of lenses makes it much easier to write new code: writing a few of these PRs I've definitely wished that this one were merged already.