Fox DHx2 or Float X2 on Nomad 4

The main upgrade with the current Nomad is its much improved leverage curve, or rather its more favourable progressivity than now caters better to coil shocks.

That being said, from what you describe increased suppleness isn't what you are after, nor necessarily more tunability - which is what the DHX2 primarily delivers. The X2 should provide a slightly more springy feel, though the tunability is pretty much the same besides the ability to more easily tune sag (air spring pressure) and spring progressiveness (air spacers). The point being, there isn't a huge performance gain to be had from going from a Super Deluxe RCT to any of the Fox offerings imo.

If you have a Super Deluxe R any of the three aforementioned shocks would be meaningful upgrade providing more tunability. It sounds like what you are after is more low speed compression adjustability.

If you focus on going downhill as fast as possible and do not mind negating a bit of adjustability, dog for a DHX2 for max suppleness. I would also go down this route if you are a set and forget type rider, only adjusting the platform for going uphill. A super deluxe RCT coil does more or less the same thing.

If you prefer a bit more poppiness while easily being able to fine tune the shock to various terrains - that is play around with air pressure and progressiveness to make the bike more trail worthy, and decrease it for full on enduro/dh mode - than the x2 is the way yo go. A super deluxe RCT does more or less the same thing. The air shocks saves you from the hassle of getting the correct weight (stiffness) spring, especially if you are between stiffnesses. By setting the bike up a bit stiffer you can make the bike slightly more inspiring when riding trail, where a plush Nomad 4 can be just too much bike.

or, fekk what i wrote above and be on the look out for a compatible (used) Push 11/6 coil shock and get the best of both worlds - if at a heavy premium. Extraordinary performance though - I cannot say enough good things about mine. Being a set and forget rider for the most part, the 11/6 matches perfectly with just the lever generating to very distinct ride feels. EXT STORIA LOK V3 supposedly delivers similar performance, if again at a premium.

And to end, kinda of a stupid point, but just crank them pedals. The rear cinematic of the n4 are very very good, and responds well to peddling to firm up the platform - aka the anti squat of the design should make the bike pedal rather well. The n4 is by miles the best Nomad design thus far, with both sorted geo and cinematic - if you have the terrain to use that much bike.