I totally understand not wanting to use your R5 underwater especially as a new underwater photographer. Underwater is a hostile environment for a camera and even the smallest amount of dirt or debris on an o-ring has the potential to compromise the seal and flood the camera & housing.
I also would not recommend trying to find a housing for your 5D Mk2. Housings tend to be made only for current cameras and so if you could find one it would be a used one that you found by pure luck.
There is, IMHO, a "middle ground" that would offer the benefits of a modern camera and housing while not being nearly as expensive as a housing for your R5. You might want to consider something like this:
Olympus OM-D E-M10 IV Camera & 14-42mm EZ Lens Kit with Backscatter Octo Underwater Housing
www.backscatter.com
Is it perfect? No, but it might be a good starting point for you at a reasonable cost. Keep in mind that you will also be a strobe (ideally a pair of strobes) in addition to this. Unlike photography above ground, underwater strobes are essential pieces of gear.
This is just a thought, but you might have options which you might not have considered.