you may not need a 3090 and as Alex points out all over this site. Meaning you might get better price per dollar on mid level cards or a machine with several 1080's for example. There's some sweet spots you can achieve. The 3080 sounds like the sweet spot. I also might be planning my next pc build with two of those hehe. But then again, I might try to put my next one together with two 1660's. You can see here.. they have the best performance per dollar.
A comprehensive List of all modern Nvidia Graphics Cards in Order of Performance in different Benchmarks such as Gaming and Rendering.
Just keep in mind (aka don't be like me) that spending the most doesn't necessarily yield the best machine.
There's also.. ALREADY some recent feedback on the new 30 series and it's not double like they were originally saying. It's good, but not like they've been selling it as.
I, like you am a long time Mac user who recently built my first pc. It's a beast, and I love it. It's not without it's issues.. still constantly ironing those out.
Most recent one is a sudden crash due to TDR error. And, what is all over this site as well, is be wary of bottlenecking. There's two 2080Ti's in my pc build and now, just within the last month I've been trying Octane X on my new iMac and it works pretty well, as does DaVinci resolve. So, not sure what your use is, but like me you sound like you want the best scorching monster of a pc and that may or may not be the best investment. Recently I asked Alex about CPU and responsiveness with the fastest processor. Don't get me wrong, GPU rendering is FANTASTIC. But, I have noticed some latency and slowness with BOTH of my current pc's. Not so much with my new iMac. Which apparently has the fastest processor and responsiveness you can get (I guess?) currently. At least until Apple silicon comes out.. maybe.. whenever that is lol.
Don't get me wrong I wholeheartedly encourage you to build one! Just don't break the bank, it might not be worth it. But, it will always be a damn good render station.
I will soon report back with some tests I'm gonna do. I'm going to pit both my top machines against each other in a friendly competition. We'll see how the $3600 pc does against the $3600 iMac, and that'll be fun. (pricing is rough estimate)