My personal opinion is that the devil is in the details. What I mean by this is that it will depend on budget, what you want to do right now plus what you want to do in the future, how willing you are to "build and rebuild" over time, how much physycal space you have to keep your rigs at, etc.
I like having full control of my devices, both at the hardware and at the software levels, and I'm well enough financially that I can change whatever I need or want without affecting my family's quality of life negatively. But I also dont just want to throw money out the window, so I research a lot when I want to add or switch something. Based on this, my preference is to DYI instead of choosing a pre-built server.
For what you say you want to do, I think a QNAP or Synology NAS would suffice (I've had both, but I like QNAP better, although they are both very similar when you compare them at the same tier levels). However, if and when you decide to up your game, you already spent the money on the pre-built, and it's not always easy to sell them used to recoup a fee bucks (which is why I chose to give mine to my sister instead of going the "selling it" route.)
I speak from my experience, and I understand that what works for me won't necessarily work for everyone else. But I like letting others know where I made kistaies in hopes that they can avoid them when the time comes.
A good example is that you want to go the Raid way, whereas I have UnRaid because I like the array option much better (I think its a more flexible approach), which can also be achieved by using OMV.
Your understanding of mybtrain of thought on this subject is exactly right. I personally prefer to have more control on as many components as possible in case I need/want to upgrade (RAM, storage, processor, GPU, TPU) because, in my experience, once I started self-hosting, I find something new I want to add almost daily (80% of the time I just test, end up not liking it, and remove, but this is how I've gotten my server to the state it's in today, trial and error mostly 🤣🤣).