I carry a lot of stuff in my trucks. I don't have a R50, but the rear suspension is very similar to my WD21 Pathfinder and my 2004 Kia Sorento.
With my Pathfinder, I went through several sets of rear springs trying to find a happy balance between lift, load capacity, and road/off road ride compliance. The Rover springs will probably be a reasonable way to go.
With my Sorento, load handling and ride quality was the criteria when the original springs got tired and sagging. Spring change was more expensive so I opted to try a set of Airlift bags. I got mine for about $80 and spent about 2 hours one evening after work installing them. I have about 2.5 years on them both on and off road. I haven't had any issues with them and still holding air just fine. I haven't had to add any air to mine since I found the balance for me. I have the air line Teed so they both have same pressure and a single valve.
Pretty much all your heavy trucks (semis, busses, dump trucks) don't run steel springs anymore, they are running air springs. Many of the semi trailers are running air springs now as well. They found that they work better for them because they are lighter and can adjust for the weight of the loads. They also have as good or better life span as well.
If you are careful with your hose routing, and install of the bags, you will have long life and no problems with them.
If it was me doing what you have planned, I would lean to the bags for their price and versatility.
As for the shocks, the Bilsteins are good, but I also really like the ProComp ES 3000. I ran a pair of them on my Pathfinder for over a decade and liked them enough that when I replaced the fronts I got a set of them. The price is just a bit more than the budget ones but they are much better and generally a bit less than the "heavy duty" ones. The only reason the rear ones on my Pathfinder got replaced with a pair of Bilsteins is that I got the shocks for free (came off a brand new Titan that got lifted when I was working for the local Nissan dealership) and the right shock lost a seal and leaked it's oil after 15 years of abuse.
I personally hate the Rancho shocks, but the set I had on my Pathfinder were the RS5000, so not adjustable. The ProComps cost less and gave a much better ride for my Pathfinder. It does have heavier rated springs front and rear than stock, but the Ranchos just seemed to be valved wrong for my truck.