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.
Learned about the importance of trailing slashes in rsync by using the -delete flag.