The BRIN index is something I might need to test, though.
Yes, check that out, it might give you some ideas. A B-tree (in whatever variant) is inherently a large index type. They're ideal for finding unique values quickly, not ideal for storing redundant values, and pretty decent at finding ranges. A BRIN (Block Range Index), as implemented in Postgres, is good for finding unique values and and ranges. But here's the thing, a BRIN index takes some absurdly small % of the space of a B-tree. You have to blink and check again to be sure you've figured it right.
How can a BRIN index be so much smaller? By throwing virtually everything out. A BRIN index doesn't store all of the values in a page, it stores the min/max value and that's it. So it's a probabilistic index (of sorts.) Is the value you're seeking on such and so page? The answer is "No" or "Maybe." That's a fast test on a very cheap data structure.
When the answer is "maybe", the full has to be loaded and scanned to determine if a specific value is found. So, you have a very small index structure, but have to do more sequential scanning to determine if a record is indexed in that page or not. In the real world, this can work out to be a high-performance structure at very low cost. But for it to work, your records need to be physically ordered (CLUSTER) by the condition in that index. And, going forward, you ought to be inserting in order too.(More-or-less.) So, a BRIN index is a great option if you have an insertion pattern that allows it to remain efficient, and if you're goal is range searching without a heavy B-tree index to maintain.
I have no clue how BRIN indexes and partitioning interact.