I eventually moved to an energy-efficient PC. It has a large case and 6 SATA ports and lots of RAM. That replaced pretty much everything I had before. I try to buy the largest hdd I can afford every time I need a new one. And the old NAS and smaller harddrives get a second use as backup space and for experimenting.

I started my selfhosted journey with a synology nas. But the nas was crap, so I ended using it only as storage and backup solution, providing NFS shares.

So, as a NAS then.

Having a separate machine as a NAS is good, running TrueNAS or XigmaNAS. You can segregate your backups from your daily use, you can have automated tasks run, etc.

Machine wise, I'm gonna try out a Beelink with nvme slots. Right now I'm using a Proliant microserver. Don't get one with a hardware RAID card as you want to be able to use zfs. But otherwise they sell pretty cheap on Ebay.

While their latest adventures into trying to force people into buying their branded hdds sucks I must say I really enjoy using Synology NAS as a storage location and their Hyper Backup software works great with rotating usb drives (setup one task per disk and disable notifications on the task if the usb drive is disconnected).
I found a Synology became a lot more responsive when I installed an extra 16GB of RAM (Crucial 16GB DDR4 3200MHz CL22 SODIMM CT16G4SFRA32A works in both Synology DS224+ and DS443+)

If I were to build a new storage solution at home today I would look at ZFS-1 and 5 large HDDs (probably 16TB drives)
https://itsfoss.com/what-is-zfs/
I'd put it in a Jonsbo N2 or similar case and combine it with an M-ITX Intel N100 integrated motherboard and a PCIe SATA Expansion Card.
Then I would continue to run two devices and have most services run on another device using the Jonsbo N2 primarily as a storage device. If you don't need to transcode 4k Blu-Ray or something then the N100 is fully capable of hosting a Jellyfin server.

Using the Synology as storage and another machine to host everything else is a perfectly fine solution, especially if you have the hardware already. If you want to consolidate everything onto one box, there are quite a few options from the likes of Ugreen, Aoostar or Minisforum. But those don't really provide much of a benefit other than having one fewer box.

I've just finished setting up exactpy what you describe (Synology NAS and Debian mini PC server) and it is working so well. Why do you need a more powerful NAS? If you use it for just storage you won't really be falling short of NAS compute power.

I considered adding the drives directly to the mini PC as well, but I'm happy I've kept the Synology for the best of both worlds. Synology makes it so much easier to share the storage, set up reverse proxy, provide constant 2 way sync with my online cloud WebDAV service, etc.