It feels to me that they’re doing a lot of consolidation either shoulder pads. I’m certain that I’m about a years time all that will be available will be the mk3 & MK4 versions in some way.
Let’s not forget that GW do produce 4 of the Legions shoulder pads in plastic. Whilst not specifically for HH marks of armour they do work very well!
Yeah. Maybe they'll do a thing where it's a bundle of Mark III/Mark IV shoulders.... Hopefully
In regards to the plastic ones: The Dark Angels ones are kind of "meh", but their Primaris Upgrade shoulders actually fit the aesthetic of the FW ones more, while being noticeably different. Blood Angels lack the defined wings. All of the other ones (Ultramarines, Space Wolves, Iron Hands, Raven Guard, Salamanders, Imperial Fists and White Scars) are a pretty good match, just not Mark III-looking (Primaris Shoulders look similar to Mark IV, which is why I was using the Iron Warriors Mark IV on my Iron Warriors Primaris Marines)