You may want to consider a mini PC. That was my upgrade after torturing my raspberry pi for many years. I landed here after agonizing over building the perfect NAS media server. Still very low on power consumption, but the compute power per dollar is great these days. All this in only a slightly larger form factor over the pi. I brought over the drives from the pi setup and was up and running for a very low cost. The workload transferred from the pi (plex, NAS, backups, many microservices/containers) leaves my system extremely bored where the pi would be begging for mercy.
I don’t do a lot of transcoding, so I’m no expert here, but looking at the documentation I believe you would want a passmark score of 2000 per each 1080p transcode, so 8000+ for your 4+ streams, not including overhead for other processes.
No worries. I got a beelink S12, non-pro model with 8G RAM and 256G SSD. It was on sale for about $150 USD. Fit my use case, but maybe not yours, although you might be surprised. Perhaps those extra plex share users won’t be concurrently transcoding?
The drives are all USB, the portable type that requires no power source. Like you, I don’t need much. I have ~12T across 3, with a small hub that could provide more ports in a pinch. This model I believe also provides a SATA slot for a 2.5” drive, but I haven’t used it. All of these drives were previously connected to a rpi 3B+, haha!
The drive shares are done via samba and also syncthing. I have no need for a unified share via mergerfs, but I did take a look at this site for some ideas. I’m the type that rolls all their own services rather than using an NAS based distro. Everything is in an ansible playbook that pushes out my configs and containers.
Edit: I should make it clear the NAS is for other systems to access the drives. Drives are directly connected via USB. All my services are contained in this single host (media/backup/microservices/etc). My Pi’s are now clustered for a k3s lab for non critical exploration.
I’m a bit of a minimalist who designs for my current use with a little room to grow. I don’t find much value in “future proofing” as I’ve never had much success in accomplishing that.