My rando bike frame came with a 130mm dropout spacing. I did not bother trying to cold set it, I am running a 135mm rear hub in it, it is easy enough to pull the stays out 5mm when I put a wheel into the frame. If your frame is a 126mm frame and you go to 130mm, I would try it first to see if you really think you need to cold set it to wider dropout spacing. It might be easy enough to set a wheel on the dropouts and pull the stays out for the wheel to drop in.

What size tire width do you plan to run? That may be a factor on whether you would prefer tubeless or not.

You said this is rear wheel you are contemplating, would you not have a front tire to match the rear? Are you planning to get both front and wheels?

I have two derailleur touring bikes, both have the same 3X8 drive train with Shimano indexed bar end shifters (BS64). I am very happy with that. But there are some on this forum that think you should go with the most modern drivetrain possible, like a 1X when making changes. I think that would be personal preference. You did not say if your bar end shifters are the older friction ones or indexed ones, or if you plan to change those too. I think that Microshift makes new 9 speed ones, but I am not sure on that.

I build my own wheels, so I can't suggest a good vendor for you to buy complete wheel(s).

I have Velocity Dyad rims on my light touring bike, very happy with that for a non-tubeless rim.