Depends on your skill level. TrueNAS is pretty great at core NAS functionality and its apps situation is also pretty good now that it's stabilizing after working through some growing pains, but it's also not as easy to use as Synology. I've used it for years (since it was still called FreeNAS) and personally wouldn't consider anything else. They offer their own hardware that is not badly priced, or you can load it up for free on your own generic DIY hardware (which will typically save you quite a few bucks over buying any all-in-one solution). Unraid is another good option for DIY that is easier to learn, though not free. Rolling your own from scratch on top of Debian or something also isn't particularly difficult if you're experienced with Linux, and running apps through Docker with something like Portainer.

For more direct competitors to Synology in terms of maximum easy user experience there are some decent options out there like QNAP, ASUStore, etc. Youtube is a good place to look for reviews that go in depth to compare.