This is what your eye will see with a 32mm Plossl and the C5 with and without the 2x Barlow. The only camera that can capture the entire Moon with a 2x Barlow and the C5 is a full frame DSLR like a Canon Eos 5D or something similar.
Here is the camera FOV of a Canon EOS 5D + 2x Barlow + the C5.
Then why use a 2X barlow?
I actually canceled the order for the 5SE:
A C5 will have a margin of only 0.01 Lunar diameters on a Canon APS-C sensor recording in 1080p.
It would have a margin of 0.1 Lunar diameters though if taken using continuous shooting and then time-lapsed.
I think a small refractor and a 2X Barlow would work out better (and more cheaply) for full-disc Solar/Lunar imaging.
Not sure I get your math...
A Canon APS-C sensor is 14.8mm tall
14.8*57.3/1250 = 0.67 degrees or just under 41 arc minutes (40.709).
The moon is roughly 31 arc minutes in angular size. 41-31 = 10 10/ 2 = 5. 5/31 = about 0.16 lunar diameters of separation on both top and bottom.
Shooting in a 16:9 mode will reduce the height of the frame by a 0.85X, I believe (3368 pixels of the 4000 vertical pixels in the sensor). This yields a 34.6 arc minute height, which is definitely tight. But even here we have 40.7 - 34.6 = 6.1 6.1/2 = 3.05 3.05/31 = about 0.05 lunar diameters.
Even with a 32 arc minute disk, I still get 0.045 lunar diameters, which is very tight, but not as tight 0.01...
Put on the F/6.3 reducer and you'll have plenty of breathing room. (0.5 lunar diameters, or 0.35 lunar diameters)
If that makes the moon too small for your tastes, you could use less than 100-105mm of spacing between the reducer and the sensor, and increase the size of the disc in the frame.
I question that a refractor solution would be cheaper AND better... You could get cheaper. You could get better. But I doubt you will find both.
An 80mm F/5 achromat would mean a cheaper OTA. But it will suffer from a lot of CA and have less resolution, so I don't see it as being better. And when you stick on a 2X barlow, you'll be at 800mm (possibly a bit more).
A 90mm F/11.1 achromat is cheap, and address much of the SA issues, but it's still not going to match the C5 for resolution, and its going to require a much more substantial mount for good results.
An 80mm ED doublet might compete in detail with the C5, but even the most affordable ones plus a suitable mount will cost more than a 5SE. And at F/7.5 (the typical focal ratio for the more affordable 80EDs), you get a 600mm focal length (too short), or 1200mm with a 2X barlow (rather close to the C5). Using a 2X barlow with a camera will probably give you more than a 2X boost, so you could end up worse off than you were with the C5.
A faster ED doublet than an F7.5 80ED, plus a 2X barlow could give you the focal length you desire, but it will cost substantially more than a C5.