Its not much different in 3dsmax, this is why I do cleanup and retopo in zbrush. I think your current GPU is good enough for this task if you say its smooth in Zbrush, its the limitation of Autodesk products (single core). I dont think you will get much improvement with a new workstation. I mean somewhat, but not huge (without knowing the exact specs of your current workstation, just guessing the generation based of your current GPU).
Edit:
For example my 1060 laptop (compared to this 3080 laptop) its similar in terms of viewport performance, a bit better on the 3080 when dealing with large scenes. In Zbrush I haven't noticed a difference at all. The only difference I notice is render times, obviously because my 1060 laptop is an 8thread and this laptop is a 16thread so double the speed on that front. I could probably run benchmark tests to double check but really when you are dealing with Autodesk software its kind of a bit bottlenecked anyway so faster the CPU single core the faster things run in general, scripts, interface etc.
In the end if you want to upgrade thats your bag, go for it if you want. I just find that selecting/deleting vertex is based on this single core speed and if you want to upgrade for that maybe you get a 30% increase, dont quote on me on the numbers exactly but sure (also depends on the CPU you buy obviously). There are also better software/plugins for this, for example in 3dsmax we have Sini software. This works in multi-thread CPU for deleting large amounts of vertices or optimization in general. You also have Rizom UV for UV workflow which is multithreaded, Zbrush which has some magic I'm not aware of but works well for photogrammetry or heavy models etc.