Great post Matt! And well, since there are no new 4090s a 5090 is nice because of the extra VRAM but the jump from a 3090 > 4090 is much larger in speed than the jump from a 4090 > 5090.
And Merijn, since you’re located in the Netherlands, a look at the Tweakers pricewatch makes sure you don’t want a 4090 since they are listed now as more expensive than the 5090. Of course that makes no sense, I would happily trade a 4090 for a 5090.
As Matt mentions, I think the difference between a € 5000 machine to a € 10000 is more noticeable than between € 10000 and € 15000.
And even for around € 5000 you can have a computer with the fastest consumer CPU from AMD/Intel, 192GB of memory and a 5090. If you would put in an extra 5090 or 2 extra 5090s the price will rise with like 2500 for a card and your render speed will double or triple.
Multiple GPUs will make it more expensive because of the custom liquid cooling needed or you need a big case and buy some AIO cooled 5090s, you still need to have the space for multiple radiators though. And multiple cars will only increase speed, not the amount of VRAM.
Instead of multiple cards in one machine you could also have multiple PCs and a network license. If nobody works behind those other PCs they can be pretty simple besides the GPU. I’ve no experience though with having multiple machines running KS over network and the network itself will always slow things down.
All the workstation kind of hardware is actually a waste of money if you are using it mainly to render and use other graphic software. A lot of suppliers will try to convince you otherwise, but it really makes no sense. It might would if you run very critical simulation software which also often uses CPU cores but for graphics the consumer grade of these days is perfect. Nvidia and SolidWorks like you also to believe otherwise, it’s not based on anything.
While memory is not that expensive and I was also orientating a bit I did notice that using for memory slots can cause instabilities if you want to run the ram at max speeds. Which means that if you like it to run full speed you are tied to 96GB of ram (2x48). As Matt says it’s not really noticeable if your ram is running at somewhat lower speeds and if you have applications that like more ram, slow but more will win from less but faster.
I do wonder if it’s actually the CPU that becomes a bottleneck with your current system or the amount of RAM. I’m still on a i9-9900K with 64GB and I notice more that if I for example try to photogrammetry software it’s my RAM what holds me back which will make it crawl in the end.
It’s also fun to ask CoPilot about hardware and what your plans are. I did it about a week ago. It actually was pretty clever when I asked I wanted to house multiple cards but it also made mistakes. Like it suggesting a motherboard that doesn’t exist. It must felt embarrassed when I asked for the website 
It did however consider my wish to have more spacing between the two first PCIe slots so the intentions were all good.
