It really depends on you. Synology is more plug and play solution which is easy to configure with the most basic applications.
If you want to save some money or get better hardware than used in Synology for the same prize than DIY NAS should be your choice. It will take some time to configure it, however, it is more customizable with a better hardware.
I built my first thing from scratch. I say "thing" because it was neither a NAS, nor a server, or a hypervisor. It had storage, standalone services, containers and VMs.
If you are unclear on what you need, DIY is probably the best approach.
I've invested a lot in my DIY machine. It's a node in a proxmox cluster. The other two nodes are mini PCs. If I was starting from scratch now, I would probably go with a prebuilt NAS and a bunch of mini office PCs for running VMs and other things.
DIY gives a lot of flexibility but you are managing every aspect of it. Borking the storage, borks your whole setup. It's a lot of fun but you need to know what you are getting yourself into.
Getting only a NAS will be insufficient once you start experimenting. Which means you will need to get another machine for hosting services/VMs. It can get expensive quickly.
If you go the DIY way, start witha hypervisor and virtualize everything else. That way you will have a more stable setup.
My DS1515+ just died and there was no way to tell what was wrong with it because there's no bios or video out. I think it may have just been drive corruption, but without a DIY build that can't be fixed
I’d recommend a look at r/Synology . My only regret is that I spent time with other devices and external multibay drive enclosures before going with it. I use it for a number of different things including a Plex server and backup for computers and phones. I’m happy with it, and prefer to avoid the tinkering aspect of DIY.