Don't worry about a ZIL/Slog or L2Arc device. I have ~100TB across a couple FreeNAS servers. One is a racked dual xeon hexacore 24-bay enclosure, and the other is a standalone little xeon quadcore 8-bay enclosure. I use Plex on both, haven't utilized anything beyond the spinning rust, and it works just fine. IF you run into issues you think can be resolved by it, then look into it. For now, don't worry about it.
64gb is plenty.
Mirroring m.2 drives for boot is a bit overkill. I'm booting from a single USB drive in my big racked NAS, and from a single m.2 in my little desk NAS. Boot from a cheap SSD or USB drive, then save your config somewhere safe. If the drive ever fails, you simply replace, reinstall, upload config. The OS loads into memory once each time you reboot (once every month or three). Mirror the m.2 drives and use them for your Plex jail if you want it to load metadata quickly. I'd even go with smaller, cheaper drives.
Transcoding doesn't need flash memory. Your CPU will be the bottleneck. Don't forget that even UHD Blu-rays can be streamed from a single HDD... If you have modern devices, you shouldn't even need transcoding within the home. I direct stream HEVC HDR blurays from my e3 Xeon over Wifi to my Sony smart tv without any issues. h.264 can be hardware-decoded by everything, and even h.265/hevc hardware decoding is pretty common. I wouldn't expect transcoding to be necessary excep when bandwidth is limited by streaming outside the home, or when streaming to old devices.
Try 1gbe before upgrading. I understand wanting 10gbit -- but wait until it's necessary. Even a 50mbps bluray direct-stream will only use ~5-10% of your 1gb link. Once you NEED 10gbit, it's likely that the prices on switches will be lower. I have 10gbit between my desktop and my little NAS just to make transfers speedy.
You will have a little under 50TB usable with that configuration. See here: https://wintelguy.com/zfs-calc.pl
What's going on with the harddrives? Would the Element drives be good for a NAS? I prefer HGST above WD, and WD above Seagate, but I would give these a chance at the current prices:
10TB, $259
12TB, $325
14TB, $388