4 wt switch should be sufficient.
A while back I converted an older 6 wt 9 1/2 SAGE SP single-hand rod to a 4wt. switch rod and found it easy to punch large ungulate hair dry flies and petrochemical floaters into the wind. I mention this because a 6 wt single hander should be more than sufficient to cover those conditions, certainly with an appropriately matched shooting head (e.g., 8wt)
Just ordered a NextCast Fall Favourite 35 330 grain line for an Echo SR 10 1/2 ft. 4wt. (This FF 35 is rated 4/5.) Haven't fished it yet but it casts very well, much better than a 270 grain Rio
light compact Scandi head I had been using. And that Scandi head was already much easier to cast while tossing large floating flies in nasty winds than a WF-6-F line on fast action 6 wt single hander.
I cannot comment on throwing 4-inch weighted streamers and bucktails for trout with a double-hand rod as I have essentially stopped fishing for trout with wet flies. A personal preference nothing more. But judging from lots of experience hucking big heavy stuff decades ago, including wind-swept Patagonia, I reckon that a 4 wt switch rod should be more than sufficient.
The trick is to load the switch rod in the mid-, bottom portion of the rod and let the rod do the work. Casting plane discipline always helps in tough winds.
If you have any Nordic blood and the resulting problems of tendinitis, and similar, you may never fish a single hander again.