Advice is all well and good, but it’s not a substitute for you taking the time to go and read up on what people are suggesting and understand why and how it relates to your needs. Start by ignoring any preconceptions you have about anything, so far they haven’t done you any favours (spec, 4K/transcoding in general, RAID type, iGPU and how it relates to PlexPass, SSD vs. USB relative to UnRAID etc).
USB is ideal for UnRAID etc. as they literally boot the OS from it, it’s the same with esxi etc. and not like booting a conventional OS in that it benefits from fast IO. 2 concurrent 1080 transcodes is 4K of CPU Mark if you need to do it in software, if you have PlexPass then you can use the iGPU (if available) to do the transcoding in hardware, but even then your CPU requirements are going to be very low. Have you checked any of the CPU’s you keep mentioning to see what the CPU mark for each is? You can also likely get away with 4GB of RAM, 8GB will give you a comfortable margin, 32 is largely pointless. Please, please go and at least try and work out why R5 is a horrible choice, it’s not the rebuild time that’s the problem. Hardware RAID is not easily expandable, if the controller fails, expect to have to replace the controller, ideally with an identical one, running the same firmware (and in some cases the same hardware revision), drive sizes being discussed mean you are almost guaranteed to get a flawed restore and the chance of successive failures while rebuilding are significantly higher, so can you tell us why you are fixated on R5? Have you even looked at what unRAID does and how it works? For a predominantly WORO based usage scenario such as yours, it’s the obvious choice.