I ended up getting the 410x410 for my Ender 3 a few months ago since I built it out with the Ender Extender kit. I also like it, and the only times I have problems with adhesion is when my filament is wet.
Are you, by chance, a software developer? It's the only other context I've heard for unit tests, and I would probably have taken a slightly different approach to it.
If I look at the cavity, I see three sections I could divide it up into. The left side with your tees, the middle section, and the far right with the curves. I would likely have modeled each of those little components, printed a section that's a few layers high to test the fit, and then once I was happy, I'd have joined them all together. Why? Because each failure is smaller, and I can iterate on my design faster.
This isn't me saying either approach is correct. Just offering an alternative perspective to consider
Are you actually using PLA as the die surface? If so, have you considered printing a version of this that will create a piece that could be used to cover the PLA for the desired size? That way you wouldn't have the PLA itself having to hold the desired shape, you could essentially have a metal cap over it that would create a piece that's your desired size.
If you're not opposed to taking inspiration from the gunsmithing world, there's a PLA printed jig out there to form sheet steel on a 10+ ton shop press. It's specific to the G3/CETME rifles. It's worth a check-out to see if there's anything that could help your design.
Honestly wicking it all up and laying fresh solder wouldn't be the worst thing. Just... don't attack it with the knife or whatever you're doing, op. This can be saved with the correct tools. A wick and some patience will carry the day.
The Revolutions is also excellent