Frame material - 853, 653, Spirit, XCR, stainless, non-stainless, whatever - will have essentially no effect on stiffness. The Young's Modulus of all steels is so close that the differences will be virtually non-existent.
The key factors are going to be wall thickness and tube diameter. Most of the tubes you've identified will have wall thicknesses in the non-butted parts of the tubes of 0.45mm or 0.5mm for the main frame tubes. So the diameter is going to be the most important factor.
If you use a 38mm Spririt downtube and some nice big chunky Max chainstays then it's definitely not going to be a noodly frame. But if you fancy going bigger there's always 44mm Spirit HSS DTs.
I think Isen mostly use Reynolds and Deda downtubes, but there's only so many combinations of the available tubesets to go around. I'm in the process of ordering another couple of 38mm Spirit downtubes for my next build, and the last one had Max chainstays too.
Frame material - 853, 653, Spirit, XCR, stainless, non-stainless, whatever - will have essentially no effect on stiffness. The Young's Modulus of all steels is so close that the differences will be virtually non-existent.
The key factors are going to be wall thickness and tube diameter. Most of the tubes you've identified will have wall thicknesses in the non-butted parts of the tubes of 0.45mm or 0.5mm for the main frame tubes. So the diameter is going to be the most important factor.
If you use a 38mm Spririt downtube and some nice big chunky Max chainstays then it's definitely not going to be a noodly frame. But if you fancy going bigger there's always 44mm Spirit HSS DTs.