I just built a NAS not too along ago, so I'll just say what I would have changed in my build. Maybe it will help you.

1. Get a server you can manage over something like ipmi or similar. Look at ASRock rack or super micro.
2. Try and get something with a lot of pci lanes or at least bifurcation. This way you can expand and use more nvmes
3. Go with nvmes first if you can. Depending on what you go with for a filesystem, will sort of depend how many you should start with.
4. If you go for 10gbe, don't use ethernet unless it's onboard, otherwise go with sfp card and switch. It runs cooler.
5. Try to find something with rdimm. Cheaper than ecc dimm and easier to find.
6. Don't forget a UPS. Protect your investment.

One thing to watch out for. Some of these server motherboards expect the smaller higher rpm fans. This means you may have to fiddle to get the fan curve corrected with normal fans.

If using something like zfs, you may want to have a bigger raid to start with. Otherwise you can do mirrored vdevs and combine them. But it can be a bit costly since you need 2 drives every time you want to expand and won't get as much space as say something like z2 (2 parity drives)

Nvmes will run cooler, longer, and faster. They also take up way less space.

Make sure you have backups. You could probably use your Synology for this or some old computer parts you have lying around. But if you do use something like trunas, it makes it very easy to backup. This server you can use platter drives.

16gh of memory is not enough. If you want to run workloads on there and if you do use something like trunas, you want as much memory as you can get for caching.

Also, just buy used. You can find cheap servers online or just the parts on like eBay or even AliExpress.

Good luck!