Scanners Advice for flat field "scanning" with Sony a7


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Somerset, UK
Hello. I am hoping and trying to get back to drawing after taking a 4 year hiatus in the next couple of months. Typically it takes me 100 to 300 hours to do one drawing because I OCD hard on ultra realism from shading and the tiniest details possible and because I draw on very large paper, A3+, typically with B2 for details, B4 for lining and small scale shading and B6 for large scale shading.
Because my papers are so big I don't have any way and no place where I can scan them accurately and safely (without bending or sectioning and blending in panoramas).
As in the past my only recourse has been making a picture of it (Olympus's HHHR at 50 MP or HR at 60 MP). I can do 32 MP on the Sony a7 Mark IV but I have one issue: non of my lenses are flat field enough to accurately render the entire drawing. My Tamy zooms have awesome magnification for macro but they all have pretty severe field curvature even stopped down to f 16.
The other problem is CA, because I draw only with grey B pencils the contrast between the paper and the drawing is very high. So high that CA is very noticeable and correcting for it in post in introduce false information into the image, degrading my archival attempts (I do plan to re-print them at same size or larger in the future as I never keep the originals).

Can ayone, especially the film scanners peeps, recommend a good and cheap macro lens for such a purpose. In know it's difficult to ask a lot from a lens, especially on a budget, but my needs would be:
*Flat field focus
*Extremely well controlled CA, as close to or APO as possible
*Under 500 £, preferably around 300 £ used/second hand.

The only lens I have found to fit most of the criteria is the Sigma 70mm f 2.8 DG DN Macro ART lens, it goes for 300 £ Like New in MPB right now.

Also, are there any ring type flashes that work with the Sony FE system thats cheap, 3rd party ones?

I may try to do a life size drawing based on 3 papers put together because I enjoy the challenge but that's a very long term project, it would be over 1.000 hours to make sure it comes out right.
Here's a couple of examples of drawings (and why normal scanning won't work):

Larisa & Catalin 21.10.2014.JPG
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Large size project, made of 3 A3 sheets of paper, on the bottom side you can even see the inline where I had to stitch multiple scans of the drawing (which I am trying to avoid).

Orianne 03.05.2017.jpg
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The last drawing I finished, I was in the UK at that time and I had no access to a scanner so I had to photograph the drawing but without a flash. (The difference in contrast and in the rendition of white is huge between the scan and the photograph)