i see on the download page there are a bunch of additional downloads for plant catalog collections and additional 3d asset collections.

https://www.bentley.com/software/e-on-software-downloads/

Are those additional add-on files cumulative? In other words, do I need to just download the latest ones under each heading (2023.3 and Extra_Content_2021.1_R6.zip) or do I need to download all of them to get all the extra add-ons?

I think you must download the whole bunch for plant catalog.

All the vue extras are not really worth it.

If you bought stuff from studio/eon there's at time limit (Bentley seems not to intersested by these niche products).

There are many other vegetations programs from other vendors using newer technology, at a price.... And the export is not really poser or Daz friendly

It's nice to play with, but quite complex.

While all that is mostly true, it depends on what you need/want really. But you might as well get everything while you can if you have decent internet. Complex yes, but one of the few programs that you don't have to learn all in order to use it. Delve into the complexities when you want later.

Since it's flora &  inorganic objects, and you really don't need morphs or animation, and although there's a number of format's to choose from, I just export out to OBJ's. I didn't have to change hardly anything with the imported surfaces either.

I picked up everything when it got released into the wild a couple years back. I had Vue a number of years ago, (their offices were in Beaverton, OR at the time, but I wasn't), but it got too spendy & needed more of a system than I could afford.

Jump ahead 30+ years & it's not only free, but I've a system now that it purrs with. The UI is very friendly in many ways, and I discovered you can simply open the presets Folder it makes, open with a Double-Click, take a look, & simply export if you like it.