SDL on Ubuntu 20.04 should be 2.0.10. I would definitely build the latest SDL and check that first. Also check setting vid_sdl_render_driver to software.
But I definitely wouldn't rule out a bug in the use of SDL_Renderer for SoftPoly. On Linux the only value proposition is that with the software render driver it does provide a pure software code path, but on systems with OpenGL or Vulkan available it's slower even for throwing the Carmack renderer to the screen (even when not using the software render driver) so I'd be surprised if it gets a whole lot of testing.
Possibly, I don't know. I know Pi's don't typically use the most up-to-date ARM chips but I also wouldn't expect it to fail on something this badly either. Even if so - I would have thought maybe the compiler would catch it.
Do also try with -march=armv8.2-a if compiling with -march=armv8-a doesn't fix the Softpoly crash when you finally set up your system. There was a couple of memory model changes introduced in the ARMv8.2-A revision.
I tested all the rendering backends (vid_preferbackend 0–3, restart after each change).
In windowed mode the window's position is half outside the screen, it cannot be moved but it can be resized. The window does not have a title bar. Despite these issues in the windowed mode the game itself works.
In full screen mode everything works fine.
Also I did not experience any crashes during testing.