Great discussion. I was experimenting exactly the same approach and thinking also about the proxy sizes referencing the AC parent/placement object size. Makes sense. I know Maxwell Render Archicad proxy object reads the geometry in and then has a percentage to simplify the 3D viewport geometry represantation (there's a choice between bounding box, vertices or polygons) but that did get slow even with quite small percentages. A bit different approach, as it doesn't use existing Archicad objects but it's own AC library object as parent. Octane approach is actually great to also produce architectural drawings and BIM models in all one go. The size match would be a bonus.
But there's a big but. I ran into my RAM limits (this happens only during reload and comes back down) with already 20 trees (1 million polygons each), so I would not recommend using this method (separate Proxies), if you have more than few proxy trees. It uses times more memory, VRAM and is times slower reloading the Octane scene, compared to exporting Archicad base geometry to some other program (Unreal, Blender are free for example) to place objects there and import back ORBX to AC where 1 proxy is placed in project 0.
I did a quick test. In my case with the same tree with 1 million polygons the reload takes:
1) 50 tree proxy parents in Archicad uses all my 64 GM RAM and goes into page file so maybe that's why it's also slower, 54 seconds. VRAM used 24 GB (RTX 3090 max capacity, but doesn't go into Out of Core).
2) 50 trees placed (Cloner with same ORBX proxy tree, Render Instances must be checked) in Cinema 4D and 1 proxy brought back to Archicad reloads scene in 5 seconds, RAM usage 11 GB, VRAM used 7 GB.
3) 50 trees with Scatter inside Archicad takes 4 seconds, but unfortunately that's not a solution incase of regular design arrangements. RAM usage 11 GB (no change during reload) and VRAM used 7 GB.
Is there any other way to make arranged Proxy use so efficient as with Scatter/1 Proxy (implementing Render Instances)?
I also agree, that Proxies placed under Materials tab is not most intuitive.