Do-It-All Nas/Plex build

Current dell r610 with dual e5504 cpus and 64gb ddr3 I got for free are starting to not cut it and my synology NAS is a slow piece of garbage with terrible UI.

Use Case:
Looking to build an unraid server for storage and transcoding for plex mostly. I also want it to be able to run a windows server VM if I want to for AD/wifi controller/monitoring, and other docker containers like Sonarr, Radarr, torrenting, etc. I want to use a QSV intel CPU of 8th gen or higher to serve up 5++ 1080p transcodes, and possibly hvec 4k content in the future, but that will probably be direct-played. I hope to have between 2-4 4-8TB drives with one parity for 10-25tb of space. This will be on my home network with a gigabit symmetrical connection and gigE throughout the house, but I could connect it via fiber SFP or 10gigE to my backbone if there became a reason to(so far no reason because I want it all in one box)

I’ve read the OTiS guide, Nas killer 4.0, Anniversery 2.0 guide threads and come up with a parts list.

CPU: i5-9400 ebay 150$
COOLER: ARCTIC Freezer 12 cpu cooler 27$
MOBO: Gigabyte b365m d3sh micro atx (1 nvme, 6 sata, 3 pcie) 75$
MEMORY: G.Skill Aegis 16GB DDR4-2666 Memory 60$ (will add more when needed)
STORAGE: Crucial P1 500gb m.2 NVME ssd 68$
CASE: Fractal Design Node 804 MicroATX case 110$
PSU: EVGA 500 W 80+ ATX PSU 50$
unraid 50$

The m.2 NVME drive will be used as an unraid cache and to store plex app data, and possibly store a single small VM in the future. My main question is, is this overkill? I’d be really happy if I could get the price down 150-200$ but I don’t want to run into a situation where I decide to add something to my server and find its not powerful enough, at least for the next ~5 years. I’m not going to be gaming on it or anything too intensive but I want it to feel fast(plex buffer times, etc). It almost seems like a celeron 4900 box with DAS would do the job a lot cheaper, but I get into this thought loop where I saw "well if I can get a MUCH better processor for only 50-100$ more, thats fine, maybe I can reduce the cost of the other parts more instead?

Organized questions:

Is this overkill? Where can I reduce the price the most without impacting my requirements?

Is this PSU sized appropriately(wattage) and is this a good value choice? I don’t know if its worth it for gold/plat/titanium PSU rating or if there are deals I can wait for like slickdeals or r/buildapcdeals.

Maybe it would be better value to go server hardware? I could use the 64gb DDR3 ram from my current box but thats about all I would want to salvage, maybe the PSU if possible(have 2).

EDIT: After doing some further research I think I want to use the fractal design node 304 case, as I don’t plan to have over 6 drives if I use 8tb+ drives. If I ever did, I would go rackmount server hardware. Any recommendations for a motherboard that fits mini ITX and supports 9th gen intel? It should have 6 SATA or a SAS connector or room for a SAS card. I’m thinking the h310. Can I use the stock cooler or should I go aftermarket for cpu.