Lens design has come a long way since the original sigma art series. Mirror versions are better and smaller.
135art was one of the last DSLR lens made by them along with the 40mm f1.4. They are really optically spectacular.
At the same time you can get the samyang 135mm f1.8 you get very close performance for lot less money and weight
Photographer Dustin Abbott shares a review of the Samyang AF 135mm F1.8 premium yet affordable telephoto portrait lens for Sony FE
dustinabbott.net
At the end of day it all depends on your priorities. For me I look at the overall package. No one is going to notice the minor differences even on a high MP bodies. If you want optically perfect lens then there are options for those too on Sony.
I don't think you'll be happy with a7riv body if you prefer larger bodies. You can add vertical grip that'll give really amazing battery life in comparison to other mirrorless brands. But vertical grip isn't at nice as they used to be on DSLRs, especially having used VG on Sony A99, A77ii, they were really nice. Even a person like me that prefers smaller bodies really liked them.
On A7 series they perform the intended job adequately but that's it.
I think in general across brands VG is a lost opportunity. They can use them to add more functionality like extra card slots for smaller bodies or extra ports etc. But they just make the bodies bigger and add more juice. Of course there is the shooting vertical orientation too.... In the past I remember they used to be able to up the frame rates for example but over time they gotten less functional imo.