With regard to the aesthetic, the design has a strong art-deco appearance (which was an art movement that had its origins in France just before WWI). There are a number of other French tank designs of the interwar period that share this styling motif - H35/39, SOMUA S-35, FCM-36, the Char B1bis, the Char D2, the Char 2C, etc. I suspect that many of the designers working on these AFVs were heavily influenced by this aesthetic as they combined form and function.
French tanks also more universally employed / displayed very elaborate camouflage patterns, many of which employed not just mimetic terrain and vegetation colors, but also bright colors (such as sky blue) with bold, curving “puzzle piece” shaped spots, often separated with black lines for even more emphasis. France was the only major power to employ such bold camouflage patterns in a nearly universal manner.
This combination of the art-deco design and bold camouflage patterns is why I think the R-35 has an almost uniquely French-look that is shared by many of its French contemporaries.
(The US cast hull M3 and M4 medium tanks share, IMO, the same art-deco styling, with the M3 cast hull medium the most pronounced non-French example that I can think of.)
The issue of using large castings for AFVs was a technology that was only just being developed and perfected. It was not as simple and easy as one might think. It did offer significant advantages in the costs of manufacturing and was also thought / believed to offer advantages in ballistic resistance to shot and shell.
Unfortunately, very large castings (like that used on the US M3 and M4 medium tanks) were very difficult to make with consistent thickness and hardening through heat treating. The mold making was also quite challenging, and only some very specialized firms were capable of manufacturing such large castings.