Of the dozens of new Subaru pans we've gone through over the years, exactly zero have been flat. I have a brand new BRZ one here that came out of the box a couple of weeks ago, and it's not flat too. What's worse is when customers remove the pan, it often get mangled and is definitely NOT flat. Some look like they've been hammered back into shape with a rock
The sealant doesn't care.
I would recommend your first thing to try is bolting down a dry OEM lower pan to the top pan, measuring how much it flange lifts between the bolts. It starts to lift as you approach target torque spec (~8ft/lbs IIRC), and gets way worse if you exceed it. Again, the sealant doesn't care.
A back plate would help maintain flatness, but you'd be needing to use a higher torque spec (higher torque rated bolts) and those are only M6 threads in the block.
Maybe a thick back plate with a gasket? O-ring has nowhere near enough room (~.09) on an FA24 unless you use an inappropriately small one.
It's a good challenge for sure, I applaud your stubbornness. We've been down this path more than a few times and don't use sealant on some unique setups. If there's any data, dimensions, or otherwise, we can provide for your journey, please don't hesitate to ask.