Those upgrades are external, they’re peripheral devices outside your iMac. The eGPU is connected by Thunderbolt and the drives could be Thunderbolt or just USB 3.0. You can get some very fast storage that uses M2.NVME cards over Thunderbolt that’s almost as fast as the internal SSD inside your iMac. If your existing iMac doesn’t have an internal SSD, getting a fast external one could speed things up for you without buying a new machine.
Also be aware that even if the eGPU can take an nvidia card, it won’t work on macOS so the only use of that would be to run Windows on your iMac in bootcamp. If you put an AMD card in it, you’re back to looking for a renderer that uses AMD GPUs. GPU rendering is usually much faster than CPU. I think the only option for GPU rendering on AMD is in blender or Indigo as someone mentioned then deleted (not sure why they deleted?).
The 27" iMac has a door on the back to let you upgrade RAM, but that’s all. If you buy the minimum 8GB from Apple, then buy your RAM from Crucial, OWC or Amazon you can save hundreds of pounds. Also, OWC has 128GB kits which gives you twice the RAM for about the same money as buying 64GB from Apple. OWC even lets you post back your old RAM afterwards (up to 60 days after) to trade it in.
You can do some upgrades internally if you’re feeling brave. I’m not up to date on this, but you used to start by removing the screen with glazier’s glass suckers, not something I’d want to do.
I’m about to buy a new Mac myself soon as this old Mac Pro has serious problems now, shutting down at random times. I’m thinking about the 27" iMac with 3.6GHz 8-core Intel Core i9 (Turbo Boost up to 5.0GHz), for the fastest single core speed with 16 threads for CPU rendering. When I bought this Mac Pro originally I opted for the most cores, but now I’d choose fastest clock speed over more cores as rendering isn’t what I spend most of my time on. Unless you’re rendering for hours a day, your computer will feel faster in normal day to day use with a faster single core speed. Of course SketchUp will be happier with a fast single core speed.
I did have GPU rendering with a GTX970 in blender and Thea but I’ll just have to give up on that moving forward as Apple and NVDIA aren’t the best of friends. Thea went subscription a year after I bought it. The version I have only works with SketchUp 2019 and I won’t be able to move that version of SketchUp to my new computer (as I’ve used 2020) so Thea and I aren’t best friends either. Maybe things will get better with AMD GPU rendering support with Metal in the future, who knows?