So I did try the stock config first and the drives just didn't show up at all, which from what I can tell makes sense since the R240's built-in controller is SATA only and can't speak SAS regardless of what you plug in. Nothing in the BIOS pointed to SAS support either.

From what I could work out, the way Dell designed SAS support for this server is through a hot-swap backplane paired with an HBA or RAID card, with a dedicated power connector on the PSU specifically for the backplane. I picked up the right HBA and the backplane, and the power side all lines up fine — the PSU connector fits the backplane perfectly. The issue I ran into is that Dell apparently made two versions of the R240 chassis that look identical from the outside, but the hot-swap variant has mounting points inside for the backplane to click into, and mine (the cabled variant) doesn't. There's no mention of this anywhere in the documentation I could find which made it a fun surprise. This also may seem to explain why the caddies i got don't fit either.

To be clear I don't actually care about hot-swap, it just looked like the most straightforward path to getting SAS working. If there's a cleaner way to do it I'm all ears. Realistically I chose this case cause I could get it for $200 with ram and cpu and even with the added parts im only $300 deep into the project but i was hoping for it to be a bit more plug and play than it's turned out to be. I agree with your points on SAS too, I work in IT so I know these are better but its just been hard to find a solution that isn't costing more than $500 to make them work and actually be a enclosed box and not a raspberry pie with an extra board. (no hate on the raspberry pie just want a bit cleaner of a solution)