A little OT and maybe a dumb question: Why are those pre-designed 8020 rigs so popular? Are there any real benefits compared to putting together your own list of stuff you need to build a rig and just order those parts? When I did this I felt this is just part of the fun. And if I forgot something, it was at least my fault
The advantage is that if you make a mistake, you could end up spending a lot and also not ordering the right parts. It's true that 8020 is very flexible and has a lot of adjustments. But if you don't order enough brackets or the right kind, you could end up spending money inefficiently or not being able to rotate brackets the right way. This will lead to you either ordering too many parts or not ordering enough parts.
Companies like Sim Lab and Sim Racing Rigs have an exact part list of what you need, and usually throw in extras (If there's no issues). Even if they miss a few parts, they'll ship them to you free of cost because it's their mistake.
Building by yourself, if you make a mistake, it's your fault.
There's also the custom mounting solutions that companies use. But most of those companies sell them separately. If you build your rig the same width, you can probably buy their mount. (Sim Lab's is 60cm wide, so is Trak Racer's and Sim Racing Rig's).
If you're confident in your ability to plan and design one, don't make a mistake with your ordering, you can save money and build one yourself. I and many others have.